4.0+The+Cell

THE CELL

The cell is one of the most basic units of life. There are millions of different types of cells. There are cells that are organisms onto themselves, such as microscopic amoeba and bacteria cells. And there are cells that only function when part of a larger organism, such as the cells that make up your body. The cell is the smallest unit of life in our bodies. In the body, there are brain cells, skin cells, liver cells, stomach cells, and the list goes on. All of these cells have unique functions and features. And all have some recognizable similarities. All cells have a 'skin', called the **plasma membrane**, protecting it from the outside environment. The cell membrane regulates the movement of water, nutrients and wastes into and out of the cell. Inside of the cell membrane are the working parts of the cell. At the center of the cell is the cell **nucleus**. The cell nucleus contains the cell's [|DNA], the genetic code that coordinates protein synthesis. In addition to the nucleus, there are many **organelles** inside of the cell - small structures that help carry out the day-to-day operations of the cell. One important cellular organelle is the **ribosome**. Ribosomes participate in protein synthesis. The [|transcription] phase of protein synthesis takes places in the cell nucleus. After this step is complete, the mRNA leaves the nucleus and travels to the cell's ribosomes, where [|translation] occurs. Another important cellular organelle is the **mitochondrion**. Mitochondria (many mitochondrion) are often referred to as the power plants of the cell because many of the reactions that produce energy take place in mitochondria. Also important in the life of a cell are the **lysosomes**. Lysosomes are organelles that contain enzymes that aid in the digestion of nutrient molecules and other materials. Below is a labelled diagram of a cell to help you identify some of these structures.

There are many different types of cells. One major difference in cells occurs between plant cells and animal cells. While both plant and animal cells contain the structures discussed above, plant cells have some additional specialized structures. Many animals have skeletons to give their body structure and support. Plants do not have a skeleton for support and yet plants don't just flop over in a big spongy mess. This is because of a unique cellular structure called the **cell wall**. The cell wall is a rigid structure outside of the cell membrane composed mainly of the polysaccharide [|cellulose]. As pictured at left, the cell wall gives the plant cell a defined shape which helps support individual parts of plants. In addition to the cell wall, plant cells contain an organelle called the **chloroplast**. The chloroplast allow The **cell** is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life.[1] Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have about 100 trillion or 1014 cells; a typical cell size is 10 µm; a typical cell mass is 1 nanogram. The largest cells are about 135 µm in the anterior horn in the spinal cord while granule cells in the cerebellum, the smallest, can be some 4 µm and the longest cell can reach from the toe to the lower brain stem (Pseudounipolar cells).[2]) The largest known cells are unfertilised ostrich egg cells which weigh 3.3 pounds.[3][4] In 1835, before the final cell theory was developed, Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.[5] The word //cell// comes from the Latin //cellula//, meaning, a small room. The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.[6] plants to harvest energy from sunlight. Specialized pigments in the chloroplast (including the common green pigment chlorophyll) absorb sunlight and use this energy to complete the chemical reaction: 6 CO2 + 6 H2O + energy (from sunlight) C6H12O6 + 6 O2 In this way, plant cells manufacture glucose and other [|carbohydrates] that they can store for later use. Organisms contain many different types of cells that perform many different functions. In the next lesson, we will examine how individual cells come together to form larger structures in the human body. For more information about cells, check out:
 * The [|Cells alive!] page for more pictures
 * Thinkquest's [|The Cell] page for more pictures and information, including a virtual cell diagram
 * The Internet Bio-Ed project's [|Cell and Cell Division] page
 * And the [|WWW Cell Biology Course]

The cells The **cell** is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have about 100 trillion or 1014 cells; a typical cell size is 10 a typical cell mass is 1 nanogram.) The largest known cells are unfertilised ostrich egg cells which weigh 3.3 pounds In 1835, before the final cell theory was developed, Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. The word //cell// comes from the Latin //cellular//, meaning, a small room. The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.

​ //​ the cells part 2//

The cell is one of the most basic units of life. There are millions of different types of cells. There are cells that are organisms onto themselves, such as microscopic amoeba and bacteria cells. And there are cells that only function when part of a larger organism, such as the cells that make up your body. The cell is the smallest unit of life in our bodies. In the body, there are brain cells, skin cells, liver cells, stomach cells, and the list goes on. All of these cells have unique functions and features. And all have some recognizable similarities. All cells have a 'skin', called the **plasma membrane**, protecting it from the outside environment. The cell membrane regulates the movement of water, nutrients and wastes into and out of the cell. Inside of the cell membrane are the working parts of the cell. At the center of the cell is the cell **nucleus**. The cell nucleus contains the cell's [|DNA], the genetic code that coordinates protein synthesis. In addition to the nucleus, there are many **organelles** inside of the cell - small structures that help carry out the day-to-day operations of the cell. One important cellular organelle is the **ribosome**. Ribosomes participate in protein synthesis. The [|transcription] phase of protein synthesis takes places in the cell nucleus. After this step is complete, the mRNA leaves the nucleus and travels to the cell's ribosomes, where [|translation] occurs. Another important cellular organelle is the **mitochondrion**. Mitochondria (many mitochondrion) are often referred to as the power plants of the cell because many of the reactions that produce energy take place in mitochondria. Also important in the life of a cell are the **lysosomes**. Lysosomes are organelles that contain enzymes that aid in the digestion of nutrient molecules and other materials. Below is a labelled diagram of a cell to help you identify some of these structures.



In 1655, the English scientist [|Robert Hooke] made an observation that would change basic biological [|theory] and research forever. While examining a dried section of cork tree with a crude [|light] microscope, he observed small chambers and named them cells. Within a decade, researchers had determined that cells were not empty but instead were filled with a watery substance called [|cytoplasm]. Over the next 175 years, research led to the formation of the cell [|theory], first proposed by the German botanist [|Matthias Jacob Schleiden] and the German physiologist Theodore Schwann in 1838 and formalized by the German researcher Rudolf Virchow in 1858. In its modern form, this theorem has four basic parts: The cell [|theory] leads to two very important generalities about cells and life as a whole: Most of the activities of a cell (repair, reproduction, etc.) are carried out via the production of [|proteins]. Proteins are large [|molecules] that are made by specific organelles within the cell using the instructions contained within its genetic material (see our [|Fats and Proteins] module). Cytology is the study of cells, and cytologists are scientists that study cells. Cytologists have discovered that all cells are similar. They are all composed chiefly of [|molecules] containing carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Although many nonliving structures also contain these [|elements], cells are different in their organization and maintenance of a boundary, their ability to regulate their own activity, and their controlled [|metabolism]. All cells contain three basic features: > > || ** Plasma Membrane ** || Apart from these three similarities, cell structure and form are very diverse and are therefore difficult to generalize. Some cells are single, independent [|units] and spend their entire existence as individual cells (these are the single-celled [|organisms] such as amoebas and bacteria). Other cells are part of multicellular organisms and cannot survive alone. One major difference among cells is the presence or absence of a [|nucleus], which is a subcellular structure that contains the genetic material. Prokaryotic cells (which include bacteria) lack a nucleus, whereas eukaryotic cells (which include protozoans, animal and plant cells) contain a nucleus. There are other major differences in cell structure and function between different types of [|organisms]. For example: There are even major differences in cells within the same [|organism], reflecting the different functions the cells serve within the organism. For example, the human body consists of trillions of cells, including some 200 different cell types that vary greatly in size, shape, and function. The smallest human cells, sperm cells, are a few micrometers wide (1/12,000 of an inch) whereas the longest cells, the neurons that run from the tip of the big toe to the spinal cord, are over a meter long in an average adult! Human cells also vary significantly in structure and function. For example: []
 * 1) The cell is the basic structural and functional [|unit] of life; all [|organisms] are composed of cells.
 * 2) All cells are produced by the division of preexisting cells (in other words, through reproduction). Each cell contains genetic material that is passed down during this process.
 * 3) All basic chemical and physiological functions - for example, repair, growth, movement, immunity, communication, and digestion - are carried out inside of cells.
 * 4) The activities of cells depends on the activities of subcellular structures within the cell (these subcellular structures include organelles, the plasma membrane, and, if present, the nucleus).
 * 1) Cells are alive. The individual cells of your organs are just as “alive” as you are, even though they cannot live independently. This [|means] cells can take [|energy] (which, depending on the cell type, can be in the form of [|light], sugar, or other compounds) and building materials (proteins, carbohydrates and fats), and use these to repair themselves and make new generations of cells (reproduction).
 * 2) The characteristics and needs of an [|organism] are in reality the characteristics and needs of the cells that make up the organism. For example, you need water because your cells need water.
 * 1) A plasma membrane consisting of a [|phospholipid] bilayer, which is a fatty membrane that houses the cell. This membrane contains several structures that allow the cell to perform necessary tasks - for example, channels that allow substances to move in and out of the cell, antigens that allow the cell to be recognized by other cells, and [|proteins] that allow cells to attach to each other.
 * [[image:http://www.visionlearning.com/library/modules/mid64/Image/VLObject-1857-030625010637.jpg width="400" height="225" caption="plasma membrane"]] ||
 * 1) A [|cytoplasm] containing cytosol and organelles. Cytosol is a fluid consisting mostly of water and dissolved nutrients, wastes, [|ions], proteins, and other [|molecules]. Organelles are small structures suspended in the cytosol. The organelles carry out the basic functions of the cell, including reproduction, [|metabolism], and protein synthesis.
 * 2) Genetic material (DNA and RNA), which carries the instructions for the production of proteins.
 * [[image:http://www.visionlearning.com/library/modules/mid64/Image/VLObject-815-021205011242.gif width="72" height="72" caption="general bacteria"]] || [[image:http://www.visionlearning.com/library/modules/mid64/Image/VLObject-816-021205011243.jpg width="72" height="72" caption="protista"]] ||
 * A Bacteria || A Protozoan ||
 * The cells of autotrophic [|organisms] (most plants and some protozoans), which can produce their own food, contain an organelle called the chloroplast that contains chlorophyll and allows the cell to produce glucose using [|light] [|energy] in the process known as [|photosynthesis].
 * The cells of plants, protists, and fungi are surrounded by a cell wall composed mostly of the carbohydrate cellulose; the cell wall helps these cells maintain their shape. Animal cells lack a cell wall but instead have a cytoskeleton, a network of long fibrous [|protein] strands that attach to the inner surface of the plasma membrane and help them maintain shape.
 * [[image:http://www.visionlearning.com/library/modules/mid64/Image/VLObject-817-021205011243.gif width="70" height="115" caption="amoebaproteus"]] || [[image:http://www.visionlearning.com/library/modules/mid64/Image/VLObject-818-021205011244.gif width="93" height="115" caption="plantcell"]] ||
 * Animal Cell || Plant Cell ||
 * Only muscle cells contain myofilaments, protein-containing structures that allow the cells to contract (shorten) and therefore cause movement.
 * Specialized cells called photoreceptors within the eye have the ability to detect [|light]. These cells contain special chemicals called pigments that can absorb light, and special organelles that can then turn the absorbed light into electrical current that is sent to the brain and is perceived as vision.

multicellular organisms. Prokaryotic cells Main article: Prokaryote

Diagram of a typical prokaryotic cellThe prokaryote cell is simpler, and therefore smaller, than a eukaryote cell, lacking a nucleus and most of the other organelles of eukaryotes. There are two kinds of prokaryotes: bacteria and archaea; these share a similar structure. A prokaryotic cell has three architectural regions: On the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells; Enclosing the cell is the cell envelope – generally consisting of a cell wall covering a plasma membrane though some bacteria also have a further covering layer called a capsule. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea). The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and finally bursting (cytolysis) from osmotic pressure against a hypotonic environment. Some eukaryote cells (plant cells and fungi cells) also have a cell wall; Inside the cell is the cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Though not forming a nucleus, the DNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids enable additional functions, such as antibiotic resistance. Eukaryotic cells Main article: Eukaryote

Diagram of a typical animal (eukaryotic) cell, showing subcellular components. Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth endoplasmic reticulum (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles within centrosomeEukaryotic cells are about 15 times wider than a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is a cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. This nucleus gives the eukaryote its name, which means "true nucleus." Other differences include: The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present. The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria also contain some DNA. Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia may thus be "viewed as sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."[7] Eukaryotes can move using motile cilia or flagella. The flagella are more complex than those of prokaryotes. taken from.http://en.wikipedia.org/wiki/Cell_(biology)

With the invention of the microscope at the beginning of the seventeenth century, it became possible to take a first glimpse at the previously invisible world of microscopic life. A bewildering array of new structures appeared before the astonished eyes of the first microscopists. The Jesuit priest Athanasius Kircher showed, in 1658, that maggots and other living creatures developed in decaying tissues. In the same period, oval red-blood corpuscles were described by the Dutch naturalist Jan Swammerdamwho also discovered that a frog embryo consists of globular particles Another new world of extraordinary variety, that of microorganisms, was revealed by the exciting investigations of another Dutchman, Antoni van Leeuwenhoek .The particles that he saw under his microscope were motile and, assuming that motility equates to life, he went on to conclude, in a letter of 9 October 1676 to the Royal Society, that these particles were indeed living organisms. In a long series of papers van Leeuwenhoek then described many specific forms of these microorganisms (which he called "animalcules"), including protozoa and other unicellular organisms

taken from: [] ​ Before we can discuss the various components of a cell, it is important to know what organism the cell comes from. There are two general categories of cells: **prokaryotes** and **eukaryotes**. || on earth"]] || It appears that life arose on earth about 4 billion years ago. The simplest of cells, and the first types of cells to evolve, were **prokaryotic cells**—organisms that lack a **nuclear membrane**, the membrane that surrounds the nucleus of a cell. **Bacteria** are the best known and most studied form of prokaryotic organisms, although the recent discovery of a second group of prokaryotes, called **archaea**, has provided evidence of a third cellular domain of life and new insights into the origin of life itself. Prokaryotes are unicellular organisms that do not develop or differentiate into multicellular forms. Some bacteria grow in filaments, or masses of cells, but each cell in the colony is identical and capable of independent existence. The cells may be adjacent to one another because they did not separate after cell division or because they remained enclosed in a common sheath or slime secreted by the cells. Typically though, there is no continuity or communication between the cells. Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies. Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of eukaryotic cells. Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called **flagella** and **pili**—proteins attached to the cell surface; a **cell envelope** consisting of a capsule, a **cell wall**, and a **plasma membrane**; and a **cytoplasmic region** that contains the **cell genome** (DNA) and ribosomes and various sorts of inclusions. || Eukaryotic organisms also have other specialized structures, called **organelles**, which are small structures within cells that perform dedicated functions. As the name implies, you can think of organelles as small organs. There are a dozen different types of organelles commonly found in eukaryotic cells. In this primer, we will focus our attention on only a handful of organelles and will examine these organelles with an eye to their role at a molecular level in the cell. The origin of the eukaryotic cell was a milestone in the evolution of life. Although eukaryotes use the same genetic code and metabolic processes as prokaryotes, their **higher level of organizational complexity** has permitted the development of truly multicellular organisms. Without eukaryotes, the world would lack mammals, birds, fish, invertebrates, mushrooms, plants, and complex single-celled organisms. || prokaryotes"]] || ||
 * Cells** are the structural and functional units of all living organisms. Some organisms, such as bacteria, are **unicellular**, consisting of a single cell. Other organisms, such as humans, are **multicellular**, or have many cells—an estimated 100,000,000,000,000 cells! Each cell is an amazing world unto itself: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities. ||
 * ===Cell Organization===
 * Cells** are the structural and functional units of all living organisms. Some organisms, such as bacteria, are **unicellular**, consisting of a single cell. Other organisms, such as humans, are **multicellular**, or have many cells—an estimated 100,000,000,000,000 cells! Each cell is an amazing world unto itself: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities. ||
 * ===Cell Organization===
 * ===Cell Organization===
 * ===Cell Organization===
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/history.gif width="350" height="221" caption="Figure 1. History of life
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/history.gif width="350" height="221" caption="Figure 1. History of life
 * Figure 1.** History of life on earth.
 * ====Prokaryotic Organisms====
 * ====Prokaryotic Organisms====
 * ====Prokaryotic Organisms====
 * ====Prokaryotic Organisms====
 * ====Eukaryotic Organisms====
 * Eukaryotes** include fungi, animals, and plants as well as some unicellular organisms. Eukaryotic cells are about 10 times the size of a prokaryote and can be as much as 1000 times greater in volume. The major and extremely significant difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a **nucleus**, a membrane-delineated compartment that houses the eukaryotic cell’s DNA. It is this nucleus that gives the eukaryote—literally, true nucleus—its name.
 * ====Eukaryotic Organisms====
 * Eukaryotes** include fungi, animals, and plants as well as some unicellular organisms. Eukaryotic cells are about 10 times the size of a prokaryote and can be as much as 1000 times greater in volume. The major and extremely significant difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a **nucleus**, a membrane-delineated compartment that houses the eukaryotic cell’s DNA. It is this nucleus that gives the eukaryote—literally, true nucleus—its name.
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/celltypes4.GIF width="450" height="190" caption="Figure 2. The cells of eukaryotes and
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/celltypes4.GIF width="450" height="190" caption="Figure 2. The cells of eukaryotes and
 * Figure 2.** Eukaryotes and prokaryotes.
 * Figure 2.** Eukaryotes and prokaryotes.

This figure illustrates a typical human cell (//eukaryote//) and a typical bacterium (//prokaryote//). The drawing on the //left// highlights the internal structures of eukaryotic cells, including the nucleus (//light blue//), the nucleolus (//intermediate blue//), mitochondria (//orange//), and ribosomes (//dark blue//). The drawing on the //right// demonstrates how bacterial DNA is housed in a structure called the nucleoid (//very light blue//), as well as other structures normally found in a prokaryotic cell, including the cell membrane (//black//), the cell wall (//intermediate blue//), the capsule (//orange//), ribosomes (//dark blue//), and a flagellum (also //black//). ||
 * ===Cell Structures: The Basics===
 * ===Cell Structures: The Basics===
 * ===Cell Structures: The Basics===

The Plasma Membrane—A Cell's Protective Coat
The outer lining of a eukaryotic cell is called the **plasma membrane**. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of proteins and lipids, fat-like molecules. Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell. A form of plasma membrane is also found in prokaryotes, but in this organism it is usually referred to as the **cell membrane**. || The **cytoskeleton** is an important, complex, and dynamic cell component. It acts to organize and maintain the cell's shape; anchors organelles in place; helps during **endocytosis**, the uptake of external materials by a cell; and moves parts of the cell in processes of growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell’s structure by directing, bundling, and aligning filaments. || Inside the cell there is a large fluid-filled space called the **cytoplasm**, sometimes called the **cytosol**. In prokaryotes, this space is relatively free of compartments. In eukaryotes, the **cytosol** is the "soup" within which all of the cell's organelles reside. It is also the home of the cytoskeleton. The cytosol contains dissolved nutrients, helps break down waste products, and moves material around the cell through a process called **cytoplasmic streaming**. The nucleus often flows with the cytoplasm changing its shape as it moves. The cytoplasm also contains many salts and is an excellent conductor of electricity, creating the perfect environment for the mechanics of the cell. The function of the cytoplasm, and the organelles which reside in it, are critical for a cell's survival. || Two different kinds of genetic material exist: **deoxyribonucleic acid** (DNA) and **ribonucleic acid** (RNA). Most organisms are made of DNA, but a few viruses have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. || Prokaryotic genetic material is organized in a simple circular structure that rests in the cytoplasm. Eukaryotic genetic material is more complex and is divided into discrete units called **genes**. Human genetic material is made up of two distinct components: the **nuclear genome** and the **mitochondrial genome**. The nuclear genome is divided into 24 linear DNA molecules, each contained in a different **chromosome**. The **mitochondrial genome** is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some very important proteins. The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs", called **organelles**, that are adapted and/or specialized for carrying out one or more vital functions. Organelles are found only in eukaryotes and are always surrounded by a protective membrane. It is important to know some basic facts about the following organelles. || The nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes and is the place where almost all DNA replication and RNA synthesis occur. The nucleus is spheroid in shape and separated from the cytoplasm by a membrane called the **nuclear envelope**. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is **transcribed**, or synthesized, into a special RNA, called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. In prokaryotes, DNA processing takes place in the cytoplasm. || Ribosomes are found in both prokaryotes and eukaryotes. The **ribosome** is a large complex composed of many molecules, including RNAs and proteins, and is responsible for processing the genetic instructions carried by an mRNA. The process of converting an mRNA's genetic code into the exact sequence of amino acids that make up a protein is called **translation**. Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes—sometimes hundreds or even thousands—can be found throughout a cell. Ribosomes float freely in the cytoplasm or sometimes bind to another organelle called the endoplasmic reticulum. Ribosomes are composed of one large and one small subunit, each having a different function during protein synthesis. || Mitochondria play a critical role in generating energy in the eukaryotic cell, and this process involves a number of complex pathways. Let's break down each of these steps so that you can better understand how food and nutrients are turned into energy packets and water. Some of the best energy-supplying foods that we eat contain complex sugars. These complex sugars can be broken down into a less chemically complex sugar molecule called **glucose**. Glucose can then enter the cell through special molecules found in the membrane, called **glucose transporters**. Once inside the cell, glucose is broken down to make adenosine triphosphate (**ATP**), a form of energy, via two different pathways. The first pathway, **glycolysis**, requires no oxygen and is referred to as **anaerobic metabolism**. Glycolysis occurs in the cytoplasm outside the mitochondria. During glycolysis, glucose is broken down into a molecule called **pyruvate**. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (**ATP**). However, only four ATP molecules can be made from one molecule of glucose in this pathway. In prokaryotes, glycolysis is the only method used for converting energy. The second pathway, called the **Kreb's cycle**, or the **citric acid cycle**, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions. Once again, the cycle begins with a glucose molecule, which during the process of glycolysis is stripped of some of its hydrogen atoms, transforming the glucose into two molecules of **pyruvic acid**. Next, pyruvic acid is altered by the removal of a carbon and two oxygens, which go on to form carbon dioxide. When the **carbon dioxide** is removed, energy is given off, and a molecule called **NAD+** is converted into the higher energy form, **NADH**. Another molecule, **coenzyme A** (CoA), then attaches to the remaining acetyl unit, forming **acetyl CoA**. The **endoplasmic reticulum (ER)** is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the **rough ER** and the **smooth ER**. The rough ER is labeled as such because it has ribosomes adhering to its outer surface, whereas the smooth ER does not. Translation of the mRNA for those proteins that will either stay in the ER or be **exported** (moved out of the cell) occurs at the ribosomes attached to the rough ER. The smooth ER serves as the recipient for those proteins synthesized in the rough ER. Proteins to be exported are passed to the Golgi apparatus, sometimes called a **Golgi body** or **Golgi complex**, for further processing, packaging, and transport to a variety of other cellular locations. One function of a lysosome is to digest foreign bacteria that invade a cell. Other functions include helping to recycle receptor proteins and other membrane components and degrading worn out organelles such as mitochondria. Lysosomes can even help repair damage to the plasma membrane by serving as a membrane patch, sealing the wound. Peroxisomes function to rid the body of toxic substances, such as hydrogen peroxide, or other metabolites and contain enzymes concerned with oxygen utilization. High numbers of peroxisomes can be found in the liver, where toxic byproducts are known to accumulate. All of the enzymes found in a peroxisome are imported from the cytosol. Each enzyme transferred to a peroxisime has a special sequence at one end of the protein, called a PTS or **peroxisomal targeting signal**, that allows the protein to be taken into that organelle, where they then function to rid the cell of toxic substances. Peroxisomes often resemble a lysosome. However, peroxisomes are self replicating, whereas lysosomes are formed in the Golgi complex. Peroxisomes also have membrane proteins that are critical for various functions, such as for importing proteins into their interiors and to proliferate and segregate into daughter cells. || For most unicellular organisms, reproduction is a simple matter of **cell duplication**, also known as **replication**. But for multicellular organisms, cell replication and reproduction are two separate processes. Multicellular organisms replace damaged or worn out cells through a replication process called **mitosis**, the division of a eukaryotic cell nucleus to produce two identical **daughter nuclei**. To reproduce, eukaryotes must first create special cells called **gametes**—eggs and sperm—that then fuse to form the beginning of a new organism. Gametes are but one of the many unique cell types that multicellular organisms need to function as a complete organism. || Most unicellular organisms create their next generation by replicating all of their parts and then splitting into two cells, a type of **asexual reproduction** called **binary fission**. This process spawns not just two new cells, but also two new organisms. Multicellullar organisms replicate new cells in much the same way. For example, we produce new skin cells and liver cells by replicating the DNA found in that cell through mitosis. Yet, producing a whole new organism requires **sexual reproduction**, at least for most multicellular organisms. In the first step, specialized cells called **gametes**—eggs and sperm—are created through a process called meiosis. Meiosis serves to reduce the chromosome number for that particular organism by half. In the second step, the sperm and egg join to make a single cell, which restores the chromosome number. This joined cell then divides and differentiates into different cell types that eventually form an entire functioning organism. || major events in mitosis"]] || ||
 * ====The Cytoskeleton—A Cell's Scaffold====
 * ====The Cytoskeleton—A Cell's Scaffold====
 * ====The Cytoskeleton—A Cell's Scaffold====
 * ====The Cytoplasm—A Cell's Inner Space====
 * ====The Cytoplasm—A Cell's Inner Space====
 * ====The Cytoplasm—A Cell's Inner Space====
 * ====Genetic Material====
 * ====Genetic Material====
 * ====Genetic Material====
 * || || Interestingly, as much as 98 percent of human DNA does not code for a specific product. || ||
 * || || Interestingly, as much as 98 percent of human DNA does not code for a specific product. || ||
 * ====Organelles====
 * ====Organelles====
 * ====Organelles====
 * ====Organelles====
 * ====The Nucleus—A Cell's Center====
 * ====The Nucleus—A Cell's Center====
 * ====The Nucleus—A Cell's Center====
 * ====The Ribosome—The Protein Production Machine====
 * ====The Ribosome—The Protein Production Machine====
 * ====The Ribosome—The Protein Production Machine====
 * ====Mitochondria and Chloroplasts—The Power Generators====
 * Mitochondria** are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. As mentioned earlier, mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell. Mitochondria have two functionally distinct membrane systems separated by a space: the outer membrane, which surrounds the whole organelle; and the inner membrane, which is thrown into folds or shelves that project inward. These inward folds are called **cristae**. The number and shape of cristae in mitochondria differ, depending on the tissue and organism in which they are found, and serve to increase the surface area of the membrane.
 * ====Mitochondria and Chloroplasts—The Power Generators====
 * Mitochondria** are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. As mentioned earlier, mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell. Mitochondria have two functionally distinct membrane systems separated by a space: the outer membrane, which surrounds the whole organelle; and the inner membrane, which is thrown into folds or shelves that project inward. These inward folds are called **cristae**. The number and shape of cristae in mitochondria differ, depending on the tissue and organism in which they are found, and serve to increase the surface area of the membrane.
 * Acetyl CoA** enters the Kreb's cycle by joining to a four-carbon molecule called **oxaloacetate**. Once the two molecules are joined, they make a six-carbon molecule called **citric acid**. Citric acid is then broken down and modified in a stepwise fashion. As this happens, hydrogen ions and carbon molecules are released. The carbon molecules are used to make more carbon dioxide. The hydrogen ions are picked up by NAD and another molecule called **flavin-adenine dinucleotide** (**FAD**). Eventually, the process produces the four-carbon oxaloacetate again, ending up where it started off. All in all, the Kreb's cycle is capable of generating from 24 to 28 ATP molecules from one molecule of glucose converted to pyruvate. Therefore, it is easy to see how much more energy we can get from a molecule of glucose if our mitochondria are working properly and if we have oxygen.
 * Chloroplasts** are similar to mitochondria but are found only in plants. Both organelles are surrounded by a double membrane with an intermembrane space; both have their own DNA and are involved in energy metabolism; and both have reticulations, or many foldings, filling their inner spaces. Chloroplasts convert light energy from the sun into ATP through a process called **photosynthesis**. ||
 * ====The Endoplasmic Reticulum and the Golgi Apparatus—Macromolecule Managers====|| || The Golgi apparatus was first described in 1898 by an Italian anatomist named Camillo Golgi. || ||
 * ====The Endoplasmic Reticulum and the Golgi Apparatus—Macromolecule Managers====|| || The Golgi apparatus was first described in 1898 by an Italian anatomist named Camillo Golgi. || ||
 * ====The Endoplasmic Reticulum and the Golgi Apparatus—Macromolecule Managers====|| || The Golgi apparatus was first described in 1898 by an Italian anatomist named Camillo Golgi. || ||
 * ====Lysosomes and Peroxisomes—The Cellular Digestive System====
 * Lysosomes** and **peroxisomes** are often referred to as the garbage disposal system of a cell. Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive **enzymes**, naturally occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. All of these enzymes work best at a **low pH**, reducing the risk that these enzymes will digest their own cell should they somehow escape from the lysosome. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system. ||
 * || What Is pH?  ||
 * The term pH derives from a combination of "p" for the word power and "H" for the symbol of the element hydrogen. pH is the negative log of the activity of hydrogen ions and represents the "activity" of hydrogen ions in a solution at a given temperature. The term activity is used because pH reflects the amount of available hydrogen ions, not the concentration of hydrogen ions. The pH scale for aqueous solutions ranges from 0 to 14 pH units, with pH 7 being neutral. A pH of less than 7 means that the solution is acidic, whereas a pH of more than 7 means that the solution is basic. || ||
 * Lysosomes** and **peroxisomes** are often referred to as the garbage disposal system of a cell. Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive **enzymes**, naturally occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. All of these enzymes work best at a **low pH**, reducing the risk that these enzymes will digest their own cell should they somehow escape from the lysosome. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system. ||
 * || What Is pH?  ||
 * The term pH derives from a combination of "p" for the word power and "H" for the symbol of the element hydrogen. pH is the negative log of the activity of hydrogen ions and represents the "activity" of hydrogen ions in a solution at a given temperature. The term activity is used because pH reflects the amount of available hydrogen ions, not the concentration of hydrogen ions. The pH scale for aqueous solutions ranges from 0 to 14 pH units, with pH 7 being neutral. A pH of less than 7 means that the solution is acidic, whereas a pH of more than 7 means that the solution is basic. || ||
 * The term pH derives from a combination of "p" for the word power and "H" for the symbol of the element hydrogen. pH is the negative log of the activity of hydrogen ions and represents the "activity" of hydrogen ions in a solution at a given temperature. The term activity is used because pH reflects the amount of available hydrogen ions, not the concentration of hydrogen ions. The pH scale for aqueous solutions ranges from 0 to 14 pH units, with pH 7 being neutral. A pH of less than 7 means that the solution is acidic, whereas a pH of more than 7 means that the solution is basic. || ||
 * ====Where Do Viruses Fit?====
 * Viruses** are not classified as cells and therefore are neither unicellular nor multicellular organisms. Most people do not even classify viruses as "living" because they lack a metabolic system and are dependent on the host cells that they infect to reproduce. Viruses have genomes that consist of either DNA or RNA, and there are examples of viruses that are either double-stranded or single-stranded. Importantly, their genomes code not only for the proteins needed to package its genetic material but for those proteins needed by the virus to reproduce during its infective cycle. ||
 * ===Making New Cells and Cell Types===
 * Viruses** are not classified as cells and therefore are neither unicellular nor multicellular organisms. Most people do not even classify viruses as "living" because they lack a metabolic system and are dependent on the host cells that they infect to reproduce. Viruses have genomes that consist of either DNA or RNA, and there are examples of viruses that are either double-stranded or single-stranded. Importantly, their genomes code not only for the proteins needed to package its genetic material but for those proteins needed by the virus to reproduce during its infective cycle. ||
 * ===Making New Cells and Cell Types===
 * ===Making New Cells and Cell Types===
 * ===Making New Cells and Cell Types===
 * ====Making New Cells====
 * ====Making New Cells====
 * ====Making New Cells====
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/Mitosis4.GIF width="460" height="167" caption="Figure 3. Overview of the
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/Mitosis4.GIF width="460" height="167" caption="Figure 3. Overview of the
 * Figure 3.** Overview of the major events in mitosis.
 * Figure 3.** Overview of the major events in mitosis.

Mitosis is the process by which the diploid nucleus (having two sets of homologous chromosomes) of a somatic cell divides to produce two daughter nuclei, both of which are still diploid. The left-hand side of the drawing demonstrates how the parent cell duplicates its chromosomes (one //red// and one //blue//), providing the daughter cells with a complete copy of genetic information. Next, the chromosomes align at the equatorial plate, and the centromeres divide. The sister chromatids then separate, becoming two diploid daughter cells, each with one //red// and one //blue// chromosome. || Every time a cell divides, it must ensure that its DNA is shared between the two daughter cells. Mitosis is the process of "divvying up" the genome between the daughter cells. To easier describe this process, let's imagine a cell with only one chromosome. Before a cell enters mitosis, we say the cell is in **interphase**, the state of a eukaryotic cell when not undergoing division. Every time a cell divides, it must first replicate all of its DNA. Because chromosomes are simply DNA wrapped around protein, the cell replicates its chromosomes also. These two chromosomes, positioned side by side, are called **sister chromatids** and are identical copies of one another. Before this cell can divide, it must separate these sister chromatids from one another. To do this, the chromosomes have to condense. This stage of mitosis is called **prophase**. Next, the nuclear envelope breaks down, and a large protein network, called the **spindle**, attaches to each sister chromatid. The chromosomes are now aligned perpendicular to the spindle in a process called **metaphase**. Next, "molecular motors" pull the chromosomes away from the metaphase plate to the spindle poles of the cell. This is called **anaphase**. Once this process is completed, the cells divide, the nuclear envelope reforms, and the chromosomes relax and decondense during **telophase**. The cell can now replicate its DNA again during interphase and go through mitosis once more. || meiosis"]] || ||
 * ====//Mitosis//====
 * ====//Mitosis//====
 * ====//Mitosis//====
 * || Cell Cycle Control and Cancer  ||
 * As cells cycle through interphase and mitosis, a surveillance system monitors the cell for DNA damage and failure to perform critical processes. If this system senses a problem, a network of signaling molecules instructs the cell to stop dividing. These so-called "checkpoints" let the cell know whether to repair the damage or initiate programmed cell death, a process called **apoptosis**. Programmed cell death ensures that the damaged cell is not further propogated. Scientists know that a certain protein, called p53, acts to accept signals provoked by DNA damage. It responds by stimulating the production of inhibitory proteins that then halt the DNA replication process. Without proper p53 function, DNA damage can accumulate unchecked. A direct consequence is that the damaged gene progresses into a cancerous state. Today, defects in p53 are associated with a variety of cancers, including some breast and colon cancers. || ||
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/Meosis4.GIF width="460" height="289" caption="Figure 4. Overview of the major events in
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/Meosis4.GIF width="460" height="289" caption="Figure 4. Overview of the major events in
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/Meosis4.GIF width="460" height="289" caption="Figure 4. Overview of the major events in
 * Figure 4.** Overview of the major events in meiosis.
 * Figure 4.** Overview of the major events in meiosis.

Meiosis, a type of nuclear division, occurs only in reproductive cells and results in a diploid cell (having two sets of chromosomes) giving rise to four haploid cells (having a single set of chromosomes). Each haploid cell can subsequently fuse with a gamete of the opposite sex during sexual reproduction. In this illustration, two pairs of homologous chromosomes enter //Meiosis I//, which results initially in two daughter nuclei, each with two copies of each chromosome. These two cells then enter //Meiosis II//, producing four daughter nuclei, each with a single copy of each chromosome. || All organisms suffer a certain number of small **mutations**, or random changes in a DNA sequence, during the process of DNA replication. These are called **spontaneous mutations** and occur at a rate characteristic for that organism. **Genetic recombination** refers more to a large-scale rearrangement of a DNA molecule. This process involves pairing between complementary strands of two parental duplex, or double-stranded DNAs, and results from a physical exchange of chromosome material. The position at which a gene is located on a chromosome is called a **locus**. In a given individual, one might find two different versions of this gene at a particular locus. These alternate gene forms are called **alleles**. During Meiosis I, when the chromosomes line up along the metaphase plate, the two strands of a chromosome pair may physically cross over one another. This may cause the strands to break apart at the crossover point and reconnect to the other chromosome, resulting in the exchange of part of the chromosome. Recombination results in a new arrangement of maternal and paternal alleles on the same chromosome. Although the same genes appear in the same order, the alleles are different. This process explains why offspring from the same parents can look so different. In this way, it is theoretically possible to have any combination of parental alleles in an offspring, and the fact that two alleles appear together in one offspring does not have any influence on the statistical probability that another offspring will have the same combination. This theory of "**independent assortment**" of alleles is fundamental to genetic inheritance. However, having said that, there is an exception that requires further discussion. The frequency of recombination is actually not the same for all gene combinations. This is because recombination is greatly influenced by the proximity of one gene to another. If two genes are located close together on a chromosome, the likelihood that a recombination event will separate these two genes is less than if they were farther apart. **Linkage** describes the tendency of genes to be inherited together as a result of their location on the same chromosome. **Linkage disequilibrium** describes a situation in which some combinations of genes or genetic markers occur more or less frequently in a population than would be expected from their distances apart. Scientists apply this concept when searching for a gene that may cause a particular disease. They do this by comparing the occurrence of a specific DNA sequence with the appearance of a disease. When they find a high correlation between the two, they know they are getting closer to finding the appropriate gene sequence. || Bacteria reproduce through a fairly simple process called **binary fission**, or the reproduction of a living cell by division into two equal, or near equal, parts. As just noted, this type of asexual reproduction theoretically results in two identical cells. However, bacterial DNA has a relatively high mutation rate. This rapid rate of genetic change is what makes bacteria capable of developing resistance to antibiotics and helps them exploit invasion into a wide range of environments. Similar to more complex organisms, bacteria also have mechanisms for exchanging genetic material. Although not equivalent to sexual reproduction, the end result is that a bacterium contains a combination of traits from two different **parental** cells. Three different modes of exchange have thus far been identified in bacteria. Because viruses are acellular and do not use ATP, they must utilize the machinery and metabolism of a host cell to reproduce. For this reason, viruses are called obligate intracellular parasites. Before a virus has entered a host cell, it is called a virion--a package of viral genetic material. **Virions**—infectious viral particles—can be passed from host to host either through direct contact or through a vector, or carrier. Inside the organism, the virus can enter a cell in various ways. **Bacteriophages**—bacterial viruses—attach to the cell wall surface in specific places. Once attached, enzymes make a small hole in the cell wall, and the virus injects its DNA into the cell. Other viruses (such as HIV) enter the host via endocytosis, the process whereby cells take in material from the external environment. After entering the cell, the virus's genetic material begins the destructive process of taking over the cell and forcing it to produce new viruses. || viruses"]] || ||
 * ====//Meiosis//====
 * Meiosis** is a specialized type of cell division that occurs during the formation of gametes. Although meiosis may seem much more complicated than mitosis, it is really just two cell divisions in sequence. Each of these sequences maintains strong similarities to mitosis.
 * Meiosis I** refers to the first of the two divisions and is often called the **reduction division**. This is because it is here that the chromosome complement is reduced from **diploid** (two copies) to **haploid** (one copy). Interphase in meiosis is identical to interphase in mitosis. At this stage, there is no way to determine what type of division the cell will undergo when it divides. Meiotic division will only occur in cells associated with male or female sex organs. **Prophase I** is virtually identical to prophase in mitosis, involving the appearance of the **chromosomes**, the development of the spindle apparatus, and the breakdown of the nuclear membrane. Metaphase I is where the critical difference occurs between meiosis and mitosis. In mitosis, all of the chromosomes line up on the metaphase plate in no particular order. In Metaphase I, the chromosome pairs are aligned on either side of the metaphase plate. It is during this alignment that the chromatid arms may overlap and temporarily fuse, resulting in what is called **crossovers**. During **Anaphase I**, the spindle fibers contract, pulling the homologous pairs away from each other and toward each pole of the cell. In **Telophase I**, a cleavage furrow typically forms, followed by **cytokinesis**, the changes that occur in the cytoplasm of a cell during nuclear division; but the nuclear membrane is usually not reformed, and the chromosomes do not disappear. At the end of Telophase I, each daughter cell has a single set of chromosomes, half the total number in the original cell, that is, while the original cell was diploid; the daughter cells are now haploid.
 * Meiosis II** is quite simply a mitotic division of each of the haploid cells produced in Meiosis I. There is no Interphase between Meiosis I and Meiosis II, and the latter begins with **Prophase II**. At this stage, a new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell. During **Metaphase II**, all of the chromosomes in the two cells align with the metaphase plate. In **Anaphase II**, the centromeres split, and the spindle fibers shorten, drawing the chromosomes toward each pole of the cell. In **Telophase II**, a cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane. The chromosomes begin to fade and are replaced by the granular chromatin, a characteristic of interphase. When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell. In the case of **male structures**, all four cells will eventually develop into **sperm cells**. In the case of the **female life cycles** in higher organisms, three of the cells will typically abort, leaving a single cell to develop into an egg cell, which is much larger than a sperm cell. ||
 * ====//Recombination—The Physical Exchange of DNA//====
 * Meiosis II** is quite simply a mitotic division of each of the haploid cells produced in Meiosis I. There is no Interphase between Meiosis I and Meiosis II, and the latter begins with **Prophase II**. At this stage, a new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell. During **Metaphase II**, all of the chromosomes in the two cells align with the metaphase plate. In **Anaphase II**, the centromeres split, and the spindle fibers shorten, drawing the chromosomes toward each pole of the cell. In **Telophase II**, a cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane. The chromosomes begin to fade and are replaced by the granular chromatin, a characteristic of interphase. When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell. In the case of **male structures**, all four cells will eventually develop into **sperm cells**. In the case of the **female life cycles** in higher organisms, three of the cells will typically abort, leaving a single cell to develop into an egg cell, which is much larger than a sperm cell. ||
 * ====//Recombination—The Physical Exchange of DNA//====
 * ====//Recombination—The Physical Exchange of DNA//====
 * ====//Recombination—The Physical Exchange of DNA//====
 * ====//Binary Fission—How Bacteria Reproduce//====
 * ====//Binary Fission—How Bacteria Reproduce//====
 * ====//Binary Fission—How Bacteria Reproduce//====
 * Conjunction** involves the direct joining of two bacteria, which allows their circular DNAs to undergo recombination. Bacteria can also undergo **transformation** by absorbing remnants of DNA from dead bacteria and integrating these fragments into their own DNA. Lastly, bacteria can exchange genetic material through a process called **transduction**, in which genes are transported into and out of the cell by bacterial viruses, called **bacteriophages**, or by **plasmids**, an autonomous self-replicating extrachromosomal circular DNA. ||
 * ====//Viral Reproduction//====
 * ====//Viral Reproduction//====
 * ====//Viral Reproduction//====
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/Viruses4.GIF width="460" height="270" caption="Figure 5. Types of
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/Viruses4.GIF width="460" height="270" caption="Figure 5. Types of
 * Figure 5.** Types of viruses.
 * Figure 5.** Types of viruses.

This illustration depicts three types of viruses: a bacterial virus, otherwise called a bacteriophage (//left center//); an animal virus (//top right//); and a retrovirus (//bottom right//). Viruses depend on the host cell that they infect to reproduce. When found outside of a host cell, viruses, in their simplest forms, consist only of genomic nucleic acid, either DNA or RNA (depicted as //blue//), surrounded by a protein coat, or capsid. || There are three different ways genetic information contained in a viral genome can be reproduced. The form of genetic material contained in the **viral capsid**, the protein coat that surrounds the nucleic acid, determines the exact replication process. Some viruses have DNA, which once inside the host cell is replicated by the host along with its own DNA. Then, there are two different replication processes for viruses containing RNA. In the first process, the viral RNA is directly copied using an enzyme called **RNA replicase**. This enzyme then uses that RNA copy as a template to make hundreds of duplicates of the original RNA. A second group of RNA-containing viruses, called the **retroviruses**, uses the enzyme reverse transcriptase to synthesize a complementary strand of DNA so that the virus's genetic information is contained in a molecule of DNA rather than RNA. The viral DNA can then be further replicated using the host cell machinery. || || || When the virus has taken over the cell, it immediately directs the host to begin manufacturing the proteins necessary for virus reproduction. The host produces three kinds of proteins: **early proteins**, enzymes used in nucleic acid replication; **late proteins**, proteins used to construct the virus coat; and **lytic proteins**, enzymes used to break open the cell for viral exit. The final viral product is assembled spontaneously, that is, the parts are made separately by the host and are joined together by chance. This self-assembly is often aided by **molecular chaperones**, or proteins made by the host that help the capsid parts come together. The new viruses then leave the cell either by exocytosis or by lysis. Envelope-bound animal viruses instruct the host's endoplasmic reticulum to make certain proteins, called **glycoproteins**, which then collect in clumps along the cell membrane. The virus is then discharged from the cell at these exit sites, referred to as exocytosis. On the other hand, bacteriophages must break open, or **lyse**, the cell to exit. To do this, the phages have a gene that codes for an enzyme called lysozyme. This enzyme breaks down the cell wall, causing the cell to swell and burst. The new viruses are released into the environment, killing the host cell in the process. || Look closely at the human body, and it is clear that not all cells are alike. For example, cells that make up our skin are certainly different from cells that make up our inner organs. Yet, all of the different cell types in our body are all **derived**, or arise, from a single, fertilized egg cell through **differentiation**. Differentiation is the process by which an unspecialized cell becomes specialized into one of the many cells that make up the body, such as a heart, liver, or muscle cell. During differentiation, certain genes are turned on, or become **activated**, while other genes are switched off, or **inactivated**. This process is intricately regulated. As a result, a differentiated cell will develop specific structures and perform certain functions. || Three basic categories of cells make up the mammalian body: **germ cells**, **somatic cells**, and **stem cells**. Each of the approximately 100,000,000,000,000 cells in an adult human has its own copy, or copies, of the genome, with the only exception being certain cell types that lack nuclei in their fully differentiated state, such as red blood cells. The majority of these cells are diploid, or have two copies of each chromosome. These cells are called **somatic cells**. This category of cells includes most of the cells that make up our body, such as skin and muscle cells. **Germ line cells** are any line of cells that give rise to **gametes**—eggs and sperm—and are continuous through the generations. **Stem cells**, on the other hand, have the ability to divide for indefinite periods and to give rise to specialized cells. They are best described in the context of normal human development. of human tissues"]] || ||
 * || Steps Associated with Viral Reproduction  ||
 * # **Attachment**, sometimes called **absorption**: The virus attaches to receptors on the host cell wall.
 * 1) **Penetration**: The nucleic acid of the virus moves through the plasma membrane and into the cytoplasm of the host cell. The capsid of a **phage**, a bacterial virus, remains on the outside. In contrast, many viruses that infect animal cells enter the host cell intact.
 * 2) **Replication**: The viral genome contains all the information necessary to produce new viruses. Once inside the host cell, the virus induces the host cell to synthesize the necessary components for its replication.
 * 3) **Assembly**: The newly synthesized viral components are assembled into new viruses.
 * 4) **Release**: Assembled viruses are released from the cell and can now infect other cells, and the process begins again.
 * 1) **Assembly**: The newly synthesized viral components are assembled into new viruses.
 * 2) **Release**: Assembled viruses are released from the cell and can now infect other cells, and the process begins again.
 * ====//Why Study Viruses?//====|| || One family of animal viruses, called the **retroviruses**, contains RNA genomes in their virus particles but synthesize a DNA copy of their genome in infected cells. Retroviruses provide an excellent example of how viruses can play an important role as models for biological research. Studies of these viruses are what first demonstrated the synthesis of DNA from RNA templates, a fundamental mode for transferring genetic material that occurs in both eukaryotes and prokaryotes. || ||
 * Viruses** are important to the study of **molecular and cellular biology** because they provide simple systems that can be used to manipulate and investigate the functions of many cell types. We have just discussed how viral replication depends on the metabolism of the infected cell. Therefore, the study of viruses can provide fundamental information about aspects of cell biology and metabolism. The rapid growth and small genome size of bacteria make them excellent tools for experiments in biology. Bacterial viruses have also further simplified the study of bacterial genetics and have deepened our understanding of the basic mechanisms of molecular genetics. Because of the complexity of an animal cell genome, viruses have been even more important in studies of animal cells than in studies of bacteria. Numerous studies have demonstrated the utility of animal viruses as probes for investigating different activities of eukaryotic cells. Other examples in which animal viruses have provided important models for biological research of their host cells include studies of **DNA replication**, **transcription**, **RNA processing**, and **protein transport**.
 * ====Deriving New Cell Types====
 * Viruses** are important to the study of **molecular and cellular biology** because they provide simple systems that can be used to manipulate and investigate the functions of many cell types. We have just discussed how viral replication depends on the metabolism of the infected cell. Therefore, the study of viruses can provide fundamental information about aspects of cell biology and metabolism. The rapid growth and small genome size of bacteria make them excellent tools for experiments in biology. Bacterial viruses have also further simplified the study of bacterial genetics and have deepened our understanding of the basic mechanisms of molecular genetics. Because of the complexity of an animal cell genome, viruses have been even more important in studies of animal cells than in studies of bacteria. Numerous studies have demonstrated the utility of animal viruses as probes for investigating different activities of eukaryotic cells. Other examples in which animal viruses have provided important models for biological research of their host cells include studies of **DNA replication**, **transcription**, **RNA processing**, and **protein transport**.
 * ====Deriving New Cell Types====
 * ====Deriving New Cell Types====
 * ====Deriving New Cell Types====
 * ====Deriving New Cell Types====
 * ====//Mammalian Cell Types//====
 * ====//Mammalian Cell Types//====
 * ====//Mammalian Cell Types//====
 * Human development** begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. Approximately 4 days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst. The **blastocyst** has an outer layer of cells, and inside this hollow sphere, there is a cluster of cells called the inner **cell mass**. The cells of the inner cell mass will go on to form virtually all of the tissues of the human body. Although the cells of the inner cell mass can form virtually every type of cell found in the human body, they cannot form an organism. Therefore, these cells are referred to as **pluripotent**, that is, they can give rise to many types of cells but not a whole organism. Pluripotent stem cells undergo further specialization into stem cells that are committed to give rise to cells that have a particular function. Examples include blood stem cells that give rise to red blood cells, white blood cells, and platelets, and skin stem cells that give rise to the various types of skin cells. These more specialized stem cells are called **multipotent**—capable of giving rise to several kinds of cells, tissues, or structures. ||
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/layoutdiff8.gif width="460" height="432" caption="Figure 6. Differentiation
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/layoutdiff8.gif width="460" height="432" caption="Figure 6. Differentiation
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/layoutdiff8.gif width="460" height="432" caption="Figure 6. Differentiation
 * Figure 6.** Differentiation of human tissues.
 * Figure 6.** Differentiation of human tissues.

Human development begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism, called the zygote (//top panel, mauve//). In the first hours after fertilization, this cell divides into identical cells. These cells then begin to specialize, forming a hollow sphere of cells, called a blastocyst (//second panel, purple//). The blastocyst has an outer layer of cells (//yellow//), and inside this hollow sphere, there is a cluster of cells called the inner cell mass (//light blue//). The inner cell mass can give rise to the germ cells—eggs and sperm—as well as cells derived from all three germ layers (ectoderm, //light blue//; mesoderm, //light green//; and endoderm, //light yellow//), depicted in the //bottom panel//, including nerve cells, muscle cells, skin cells, blood cells, bone cells, and cartilage. || DNA replication"]] || ||
 * Reproduced with permission from the Office of Science Policy, the National Institutes of Health. ||
 * ===The Working Cell: DNA, RNA, and Protein Synthesis=== ||
 * ====DNA Replication====
 * DNA replication**, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job. In the first step of replication, a special protein, called a **helicase**, unwinds a portion of the parental DNA double helix. Next, a molecule of **DNA polymerase**—a common name for two categories of enzymes that influence the synthesis of DNA— binds to one strand of the DNA. DNA polymerase begins to move along the DNA strand in the 3' to 5' direction, using the single-stranded DNA as a template. This newly synthesized strand is called the **leading strand** and is necessary for forming new nucleotides and reforming a double helix. Because DNA synthesis can only occur in the 5' to 3' direction, a second DNA polymerase molecule is used to bind to the other template strand as the double helix opens. This molecule synthesizes discontinuous segments of polynucleotides, called **Okazaki fragments**. Another enzyme, called **DNA ligase**, is responsible for stitching these fragments together into what is called the **lagging strand**. ||  ||
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/dnareplication4.GIF width="300" height="274" caption="Figure 7. An overview of
 * DNA replication**, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job. In the first step of replication, a special protein, called a **helicase**, unwinds a portion of the parental DNA double helix. Next, a molecule of **DNA polymerase**—a common name for two categories of enzymes that influence the synthesis of DNA— binds to one strand of the DNA. DNA polymerase begins to move along the DNA strand in the 3' to 5' direction, using the single-stranded DNA as a template. This newly synthesized strand is called the **leading strand** and is necessary for forming new nucleotides and reforming a double helix. Because DNA synthesis can only occur in the 5' to 3' direction, a second DNA polymerase molecule is used to bind to the other template strand as the double helix opens. This molecule synthesizes discontinuous segments of polynucleotides, called **Okazaki fragments**. Another enzyme, called **DNA ligase**, is responsible for stitching these fragments together into what is called the **lagging strand**. ||  ||
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/dnareplication4.GIF width="300" height="274" caption="Figure 7. An overview of
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/dnareplication4.GIF width="300" height="274" caption="Figure 7. An overview of
 * Figure 7.** An overview of DNA replication.
 * Figure 7.** An overview of DNA replication.

Before a cell can divide, it must first duplicate its DNA. This figure provides an overview of the DNA replication process. In the first step, a portion of the double helix (//blue//) is unwound by a helicase. Next, a molecule of DNA polymerase (//green//) binds to one strand of the DNA. It moves along the strand, using it as a template for assembling a leading strand (//red//) of nucleotides and reforming a double helix. Because DNA synthesis can only occur 5' to 3', a second DNA polymerase molecule (also //green//) is used to bind to the other template strand as the double helix opens. This molecule must synthesize discontinuous segments of polynucleotides (called //Okazaki Fragments//). Another enzyme, //DNA Ligase// (//yellow//), then stitches these together into the lagging strand. || The average human chromosome contains an enormous number of nucleotide pairs that are copied at about 50 base pairs per second. Yet, the entire replication process takes only about an hour. This is because there are many **replication origin sites** on a eukaryotic chromosome. Therefore, replication can begin at some origins earlier than at others. As replication nears completion, "bubbles" of newly replicated DNA meet and fuse, forming two new molecules. With multiple replication origin sites, one might ask, how does the cell know which DNA has already been replicated and which still awaits replication? To date, two **replication control mechanisms** have been identified: one positive and one negative. For DNA to be replicated, each replication origin site must be bound by a set of proteins called the **Origin Recognition Complex**. These remain attached to the DNA throughout the replication process. Specific accessory proteins, called **licensing factors**, must also be present for initiation of replication. Destruction of these proteins after initiation of replication prevents further replication cycles from occurring. This is because licensing factors are only produced when the nuclear membrane of a cell breaks down during mitosis. || Although much is known about transcript processing, the signals and events that instruct RNA polymerase to stop transcribing and drop off the DNA template remain unclear. Experiments over the years have indicated that processed eukaryotic messages contain a **poly(A) addition signal** (AAUAAA) at their 3' end, followed by a string of adenines. This poly(A) addition, also called the **poly(A) site**, contributes not only to the addition of the poly(A) tail but also to transcription termination and the release of RNA polymerase from the DNA template. Yet, transcription does not stop here. Rather, it continues for another 200 to 2000 bases beyond this site before it is aborted. It is either before or during this termination process that the nascent transcript is **cleaved**, or cut, at the poly(A) site, leading to the creation of two RNA molecules. The upstream portion of the newly formed, or **nascent**, RNA then undergoes further modifications, called **post-transcriptional modification**, and becomes mRNA. The downstream RNA becomes unstable and is rapidly degraded. Although the importance of the poly(A) addition signal has been established, the contribution of sequences further downstream remains uncertain. A recent study suggests that a defined region, called the **termination region**, is required for proper transcription termination. This study also illustrated that transcription termination takes place in two distinct steps. In the first step, the nascent RNA is cleaved at specific subsections of the termination region, possibly leading to its release from RNA polymerase. In a subsequent step, RNA polymerase disengages from the DNA. Hence, RNA polymerase continues to transcribe the DNA, at least for a short distance. || The cellular machinery responsible for synthesizing proteins is the **ribosome**. The ribosome consists of structural RNA and about 80 different proteins. In its inactive state, it exists as two subunits: a **large subunit** and a **small subunit**. When the small subunit encounters an mRNA, the process of **translating** an mRNA to a protein begins. In the large subunit, there are two sites for amino acids to bind and thus be close enough to each other to form a bond. The "**A site**" accepts a new **transfer RNA**, or tRNA—the adaptor molecule that acts as a translator between mRNA and protein—bearing an amino acid. The "**P site**" binds the tRNA that becomes attached to the growing chain. As we just discussed, the adaptor molecule that acts as a translator between mRNA and protein is a specific RNA molecule, the tRNA. Each tRNA has a specific **acceptor site** that binds a particular triplet of nucleotides, called a **codon**, and an **anti-codon site** that binds a sequence of three unpaired nucleotides, the **anti-codon**, which can then bind to the the codon. Each tRNA also has a specific **charger protein**, called an **aminoacyl tRNA synthetase**. This protein can only bind to that particular tRNA and attach the correct amino acid to the acceptor site. The **start signal** for translation is the codon ATG, which codes for methionine. Not every protein necessarily starts with methionine, however. Oftentimes this first amino acid will be removed in later processing of the protein. A tRNA charged with methionine binds to the translation start signal. The large subunit binds to the mRNA and the small subunit, and so begins **elongation**, the formation of the polypeptide chain. After the first charged tRNA appears in the A site, the ribosome shifts so that the tRNA is now in the P site. New charged tRNAs, corresponding the codons of the mRNA, enter the A site, and a bond is formed between the two amino acids. The first tRNA is now released, and the ribosome shifts again so that a tRNA carrying two amino acids is now in the P site. A new charged tRNA then binds to the A site. This process of elongation continues until the ribosome reaches what is called a **stop codon**, a triplet of nucleotides that signals the termination of translation. When the ribosome reaches a stop codon, no aminoacyl tRNA binds to the empty A site. This is the ribosome signal to break apart into its large and small subunits, releasing the new protein and the mRNA. Yet, this isn't always the end of the story. A protein will often undergo further modification, called **post-translational modification**. For example, it might be cleaved by a protein-cutting enzyme, called a protease, at a specific place or have a few of its amino acids altered. || transcription and translation"]] || ||
 * ====DNA Transcription—Making mRNA====
 * DNA transcription** refers to the synthesis of RNA from a DNA template. This process is very similar to DNA replication. Of course, there are different proteins that direct transcription. The most important enzyme is **RNA polymerase**, an enzyme that influences the synthesis of RNA from a DNA template. For transcription to be initiated, RNA polymerase must be able to recognize the beginning sequence of a gene so that it knows where to start synthesizing an mRNA. It is directed to this initiation site by the ability of one of its subunits to recognize a specific DNA sequence found at the beginning of a gene, called the **promoter sequence**. The promoter sequence is a unidirectional sequence found on one strand of the DNA that instructs the RNA polymerase in both where to start synthesis and in which direction synthesis should continue. The RNA polymerase then unwinds the double helix at that point and begins synthesis of a RNA strand complementary to one of the strands of DNA. This strand is called the **antisense** or **template** strand, whereas the other strand is referred to as the **sense** or coding strand. Synthesis can then proceed in a unidirectional manner.
 * ====DNA Transcription—Making mRNA====
 * DNA transcription** refers to the synthesis of RNA from a DNA template. This process is very similar to DNA replication. Of course, there are different proteins that direct transcription. The most important enzyme is **RNA polymerase**, an enzyme that influences the synthesis of RNA from a DNA template. For transcription to be initiated, RNA polymerase must be able to recognize the beginning sequence of a gene so that it knows where to start synthesizing an mRNA. It is directed to this initiation site by the ability of one of its subunits to recognize a specific DNA sequence found at the beginning of a gene, called the **promoter sequence**. The promoter sequence is a unidirectional sequence found on one strand of the DNA that instructs the RNA polymerase in both where to start synthesis and in which direction synthesis should continue. The RNA polymerase then unwinds the double helix at that point and begins synthesis of a RNA strand complementary to one of the strands of DNA. This strand is called the **antisense** or **template** strand, whereas the other strand is referred to as the **sense** or coding strand. Synthesis can then proceed in a unidirectional manner.
 * ====Protein Translation—How Do Messenger RNAs Direct Protein Synthesis?====
 * ====Protein Translation—How Do Messenger RNAs Direct Protein Synthesis?====
 * ====Protein Translation—How Do Messenger RNAs Direct Protein Synthesis?====
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/proteinsynth4.GIF width="250" height="410" caption="Figure 8. An overview of
 * || [[image:http://www.ncbi.nlm.nih.gov/About/primer/images/proteinsynth4.GIF width="250" height="410" caption="Figure 8. An overview of
 * Figure 8.** An overview of transcription and translation.
 * Figure 8.** An overview of transcription and translation.

This drawing provides a graphic overview of the many steps involved in transcription and translation. Within the nucleus of the cell (//light blue//), genes (DNA, //dark blue//) are transcribed into RNA. This RNA molecule is then subject to post-transcriptional modification and control, resulting in a mature mRNA molecule (//red//) that is then transported out of the nucleus and into the cytoplasm (//peach//), where it undergoes translation into a protein. mRNA molecules are translated by ribosomes (//purple//) that match the three-base codons of the mRNA molecule to the three-base anti-codons of the appropriate tRNA molecules. These newly synthesized proteins (//black//) are often further modified, such as by binding to an effector molecule (//orange//), to become fully active. || Maintenance of the accuracy of the DNA genetic code is critical for both the long- and short-term survival of cells and species. Sometimes, normal cellular activities, such as duplicating DNA and making new gametes, introduce changes or mutations in our DNA. Other changes are caused by exposure of DNA to chemicals, radiation, or other adverse environmental conditions. No matter the source, genetic mutations have the potential for both positive and negative effects on an individual as well as its species. A positive change results in a slightly different version of a gene that might eventually prove beneficial in the face of a new disease or changing environmental conditions. Such beneficial changes are the cornerstone of evolution. Other mutations are considered **deleterious**, or result in damage to a cell or an individual. For example, errors within a particular DNA sequence may end up either preventing a vital protein from being made or encoding a defective protein. It is often these types of errors that lead to various disease states. The potential for DNA damage is counteracted by a vigorous surveillance and repair system. Within this system, there are a number of enzymes capable of repairing damage to DNA. Some of these enzymes are specific for a particular type of damage, whereas others can handle a range of mutation types. These systems also differ in the degree to which they are able to restore the normal, or **wild-type**, sequence. || || || Understanding what makes up a cell and how that cell works is fundamental to all of the biological sciences. Appreciating the similarities and differences between cell types is particularly important to the fields of cell and molecular biology. These fundamental similarities and differences provide a unifying theme, allowing the principles learned from studying one cell type to be extrapolated and generalized to other cell types. Perhaps the most fundamental property of all living things is their ability to reproduce. All cells arise from pre-existing cells, that is, their genetic material must be replicated and passed from parent cell to progeny. Likewise, all multicellular organisms inherit their genetic information specifying structure and function from their parents. The next section of the genetics primer, **[|What is a Genome]**, details how genetic information is replicated and transmitted from cell to cell and organism to organism. ||
 * ====DNA Repair Mechanisms====
 * ====DNA Repair Mechanisms====
 * ====DNA Repair Mechanisms====
 * || Categories of DNA Repair Systems  ||
 * * **Photoreactivation** is the process whereby genetic damage caused by ultraviolet radiation is reversed by subsequent illumination with visible or near-ultraviolet light.
 * **Nucleotide excision repair** is used to fix DNA lesions, such as single-stranded breaks or damaged bases, and occurs in stages. The first stage involves recognition of the damaged region. In the second stage, two enzymatic reactions serve to remove, or excise, the damaged sequence. The third stage involves synthesis by DNA polymerase of the excised nucleotides using the second intact strand of DNA as a template. Lastly, DNA ligase joins the newly synthesized segment to the existing ends of the originally damaged DNA strand.
 * **Recombination repair**, or **post-replication repair**, fixes DNA damage by a strand exchange from the other daughter chromosome. Because it involves homologous recombination, it is largely error free.
 * **Base excision repair** allows for the identification and removal of wrong bases, typically attributable to **deamination**—the removal of an amino group (NH2)—of normal bases as well as from chemical modification.
 * **Mismatch repair** is a multi-enzyme system that recognizes inappropriately matched bases in DNA and replaces one of the two bases with one that "matches" the other. The major problem here is recognizing which of the mismatched bases is incorrect and therefore should be removed and replaced.
 * **Adaptive/inducible** repair describes several protein activities that recognize very specific modified bases. They then transfer this modifying group from the DNA to themselves, and, in doing so, destroy their own function. These proteins are referred to as **inducible** because they tend to regulate their own synthesis. For example, exposure to modifying agents induces, or turns on, more synthesis and therefore adaptation.
 * **SOS repair** or **inducible error-prone repair** is a repair process that occurs in bacteria and is induced, or switched on, in the presence of potentially lethal stresses, such as UV irradiation or the inactivation of genes essential for replication. Some responses to this type of stress include mutagenesis—the production of mutations—or cell elongation without cell division. In this type of repair process, replication of the DNA template is extremely inaccurate. Obviously, such a repair system must be a desperate recourse for the cell, allowing replication past a region where the wild-type sequence has been lost.
 * **SOS repair** or **inducible error-prone repair** is a repair process that occurs in bacteria and is induced, or switched on, in the presence of potentially lethal stresses, such as UV irradiation or the inactivation of genes essential for replication. Some responses to this type of stress include mutagenesis—the production of mutations—or cell elongation without cell division. In this type of repair process, replication of the DNA template is extremely inaccurate. Obviously, such a repair system must be a desperate recourse for the cell, allowing replication past a region where the wild-type sequence has been lost.
 * ===From Cells to Genomes===
 * ===From Cells to Genomes===
 * ===From Cells to Genomes===

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