4.2+Cell+reproduction

media type="custom" key="6046513" width="254" height="150" align="center" Protein synthesis carried out Chromatin present Nucleolus present DNA replicated towards division time || Nuclear membrane disintegrates Centriole pairs move to opposite ends of the cell Spindle fibers begin to form || line up along the center of the spindle structure || Once separated, they are called daughter chromosomes Due to pull, many chromosomes bend Groove in plasma membrane present || Spindle disintegrates Nuclear membrane takes shape again Centrioles replicate Membrane continues to pinch inward (in plant cells a new cell wall is laid) ||
 * || This process of cellular division is unique to eukaryotic cells. The following illustrations depict only the replication of chromosomes and their division. Keep in mind that the process of mitosis involves the entire cell which includes a multitude of organelles. ||  ||
 * || **Interphase** || Time between divisions
 * [[image:http://library.thinkquest.org/12413/img/mitosis1.jpg width="119" height="119" caption="Interphase"]] ||^  ||   ||  || **Prophase** || Chromatin thickens into chromosomes
 * [[image:http://library.thinkquest.org/12413/img/mitosis2.jpg width="119" height="119" caption="Prophase"]] ||^  ||   ||
 * || **Metaphase** || Guided by the spindle fibers, the chromosome pairs
 * ^  || [[image:http://library.thinkquest.org/12413/img/mitosis3.jpg width="119" height="119" caption="Metaphase"]] ||   ||
 * || **Anaphase** || The chromosome pairs (sisters) begin to pull apart
 * [[image:http://library.thinkquest.org/12413/img/mitosis4.jpg width="119" height="119" caption="Anaphase"]] ||^  ||   ||  || **Telophase** || Chromosomes return to chromatin
 * [[image:http://library.thinkquest.org/12413/img/mitosis5.jpg width="119" height="119" caption="Telophase"]] ||^  ||   ||
 * || When the process is complete, each cell will have the same genetic material that the original cell had before replication. Each of the daughter cells is also identical to each other. Note that once telophase is complete, the cell returns to interphase. ||  ||

//** Cell Reproduction **//

Most human cells are frequently reproduced and replaced during the life of an individual. However, the process varies with the kind of cell. **Somatic**, or body cells, such as those that make up skin, hair, and muscle, are duplicated by **mitosis**. The ** sex cells **, sperm and ova, are produced by **meiosis** in special tissues of male testes and female ovaries. Since the vast majority of our cells are somatic, mitosis is the most common form of cell replication.  The cell division process that produces new cells for growth, repair, and the general replacement of older cells is called mitosis. In this process, a somatic cell divides into two complete new cells that are identical to the original one. Human somatic cells go through the 6 phases of mitosis in 1/2 to 1 1/2 hours, depending on the kind of tissue being duplicated.
 * Mitosis **


 * [[image:images/mitosis.gif width="499" height="203" align="center" caption="drawings of the 6 phases of mitosis"]]

||

Cell reproduction


 * The term cell growth is used in the contexts of cell development and cell division (reproduction) when used in the context of cell division, it refers to growth of cell populations, where one cell (the "mother cell") grows and divides to produce two " daughter cells ". **

Cell populations

 * Cell populations go through a particular type of exponential growth called doubling. Thus, each generation of cells should be twice as numerous as the previous generation. However, the number of generations only gives a maximum figure as not all cells survive in each generation. **

Yeast cell size regulation

 * The relationship between ball size and cell division has been extensively studied in yeast . For some cells, there is a mechanism by which cell division is not initiated until a cell has reached a certain size. If the nutrient supply is restricted (after time t = 2 in the diagram, below), and the rate of increase in cell size is slowed, the time period between cell divisions is increased. Yeast cell size mutants were isolated that begin cell division before reaching the normal size (//wee// mutants). The Wee1 protein is a tyrosine kinase . It normally phosphorylates the Cdc2 cell cycle regulatory protein (cyclic-dependent kinase-1, CDK1 ) on a tyrosine residue. This covalent modification of the molecular structure of Cdc2 inhibits the enzymatic activity of Cdc2 and prevents cell division. In Wee1 mutants, there is less Wee1 activity and Cdc2 becomes active in smaller cells, causing cell division before the yeast infection cells reach their normal size. Cell division may be regulated in part by dilution of Wee1 protein in cells as they grow larger. **

Introduction
Previous chapters looked at a cell's genetic material and main principles of inheritance. When cells reproduce, the genetic material is distributed between the newly-produced cells, called daughter cells. This topic looks at different types of cell reproduction.

Mitosis
Somatic cells and gametes reproduce differently. Somatic cells divide by mitosis. Mitosis is the process by which the parent cell divides into two identical daughter cells. These daughter cells are genetically identical to their parent cell. Mitosis is the type of cell division that leads to the growth and development of an organism. All cells, except gametes, divide by mitosis. The process of mitosis can be divided into the following stages. //See image 1.//

1. Interphase
Before actual cell division occurs, the cell grows in size. This growth stage can take a few hours or a few months. DNA molecules replicate but are still in the form of chromatin. The cell's nucleus is still intact.

2. Prophase
DNA molecules are more condensed and coiled. The nucleus is no longer a membrane-bound organelle. A spindle apparatus starts forming, which is also called a mitotic spindle. It is called that because all the microtubules attached to centrosomes at opposite poles of the cell, make it look like a spindle. The mitotic spindle helps chromosomes move in opposite directions. It is formed by the centrosome - the non-membrane bound organelle of the animal cell. Spindle micro-tubules 'drag' chromosomes apart to the newly-formed daughter cells. //See image 2.//

3. Metaphase
Spindle microtubules attach themselves to the chromosome centromeres. Chromosomes get aligned along the cellular 'equator'.

4. Anaphase
The micro-tubules of the spindle shorten and separate and drag sister chromatids towards the opposite poles of the cell.

5. Telophase
At this stage, chromosomes reach the opposite poles of the parent cell and the nuclear envelope reforms. Chromosomes uncoil and turn into chromatin again. The spindle fibres disintegrate. The parent cell splits into two daughter cells in the process called cytokinesis. Each daughter cell is diploid and contains the same type of chromosomes as the parent cell. The process of mitosis can happen in any part of the body.

Meiosis
A cell division, in which eggs and sperm are produced, is called meiosis. Meiosis takes place in all sexually reproducing organisms. In the process of meiosis, the number of chromosomes is reduced by half, so all gametes are haploid cells (1n). Sometimes meiosis is also called reductive cell division. During fertilisation gametes fuse, forming a diploid zygote: 1n+1n=2n. Meiosis consists of two, consecutive nuclear divisions with only one round of DNA replication. At the end of meiosis, four non-identical haploid daughter cells are produced. Meiosis consists of two stages: meiosis I and meiosis II. //See image 3.//

1. Interphase 1
As in mitosis, at this stage the cell grows in size getting ready for division. Chromosomes are duplicated, but are still in the cell's nucleus in the form of chromatin.

2. Prophase 1
DNA molecules are more condensed and visible. The key, new events (compared to mitosis,) are the formation of tetrads and a crossover. Each tetrad is composed of four chromatids. Non-sister chromatids of the homologous chromosomes exchange pieces of DNA in the process of crossing over. Later, the envelope of the nucleus breaks down and tetrads migrate to the opposite cell poles.

3. Metaphase 1
Tetrads line up along the cell's equator with their centromeres facing the opposite cell poles.

4. Anaphase 1
Chromosomes move to the opposite cell poles with the help of centrosome micro-tubules. The homologous chromosomes move to the opposite cell poles, yet sister chromatids remain together.

5. Telophase 1
Meiotic spindles continue to move the homologous chromosomes to the poles. Once movement is complete, each pole has a haploid number of chromosomes. The chromosomes are double stranded. Two genetically non-identical daughter cells are formed in the process of cytokinesis. The genetic material does not replicate again.

Meiosis II
There is no interphase stage in meiosis II. It starts with prophase.

1. Prophase 2
The nuclear envelope breaks and the spindle apparatus forms. Chromosomes begin to migrate towards the cell's equator.

2. Metaphase 2
Chromosomes line up at the cell's equator with their centromeres facing opposite poles of the cell.

3. Anaphase 2
Sister chromatids separate and move toward opposite cell poles.

4. Telophase 2
The cellular nucleus is formed and daughter cells are formed after cytokinesis is complete. At the end of meiosis II, there are four daughter cells, each with half the number of chromosomes of the original parent cell. Each daughter cell gets one of the four chromatids from the tetrads. The process of meiosis can happen only in organs which produce sex cells. It is possible to see different stages of cell division through the microscope because condensed chromosomes stain well with biological dyes.

Cell division
reproduction that either involve binary fission, mitosis, or meiosis. The diagram below depicts the similarities and differences of these three types of cell reproduction. **
 * Cell reproduction is asexual . For most of the constituents of the cell, growth is a steady, continuous process, interrupted only briefly at M phase when the nucleus and then the cell divide in two. **
 * The process of cell division, called cell cycle, has four major parts called phases. The first part, called G1 phase is marked by synthesis of various enzymes that are required for DNA replication. The second part of the cell cycle is the S phase, where DNA replication produces two identical sets of chromosomes . The third part is the G2 phase. Significant protein synthesis occurs during this phase, mainly involving the production of microtubules , which are required during the process of division, called mitosis . The fourth phase, M phase, consists of nuclear division ( karyokinesis ) and cytoplasmic division ( cytokinesis ), accompanied by the formation of a new cell membrane . This is the physical division of "mother" and "daughter" cells. The M phase has been broken down into several distinct phases, sequentially known as prophase , prometaphase , metaphase , anaphase and telophase leading to cytokinesis. **
 * Cell division is more complex in eukaryotes than in other organisms. Prokaryotic cells such as bacterial cells reproduce by binary fission, a process that includes DNA replication, chromosome segregation, and cytokinesis. Eukaryotic cell division either involves mitosis or a more complex process called meiosis . Mitosis and meiosis are sometimes called the two " nuclear division" processes. Binary fission is similar to eukaryotic cell reproduction that involves mitosis. Both lead to the production of two daughter cells with the same number of chromosomes as the parental cell. Meiosis is used for a special cell reproduction process of diploid organisms. It produces four special daughter cells ( gametes ) which have half the normal cellular amount of DNA. A male and a female gamete can then combine to produce a zygote , a cell which again has the normal amount of chromosomes. **
 * The rest of this article is a comparison of the main features of the three types of cell

For simple unicellular organisms[nb 1] such as the ameboa, one cell division is equivalent to reproduction-- an entire new organism is created. On a larger scale, mitotic cell division can create progeny from multicellular organisms, such as plants that grow from cuttings. Cell division also enables asexually reproducing organisms to develop from the one-celled zygote, which itself was produced by cell division from gametes. And after growth, cell division allows for continual construction and repair of the organism.[1] A human being's body experiences about 10,000 trillion cell divisions in a lifetime.[2] The primary concern of cell division is the maintenance of the original cell's genome. Before division can occur, the genomic information which is stored in chromosomes must be replicated, and the duplicated genome separated cleanly between cells. A great deal of cellular infrastructure is involved in keeping genomic information consistent between "generations".
 * Cell division** is the process by which a //parent cell// divides into two or more //daughter cells//. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort of cell division in prokaryotes is known as binary fission. In another type of cell division present only in eukaryotes, called meiosis, a cell is permanently transformed into a gamete and cannot divide again until fertilization. Right before the parent cell splits, it undergoes DNA replication.

hide] * 1 Variants
 * ==Contents==
 * 2 Degradation
 * 3 See also
 * 4 Notes
 * 4.1 Other notes
 * 5 References
 * 6 External links ||

edit] Variants
Three types of cell division Furthermore,the pattern of cell division that transforms eukaryotic stem cells into gametes (sperm in males or ova in females) is different from that of eukaryotic somatic (non-germ) cells
 * Cells** are classified into two categories: simple, non-nucleated prokaryotic cells, and complex, nucleated eukaryotic cells. By dint of their structural differences, **eukaryotic** and **prokaryotic** cells do not divide in the same way.