- When cells divide, the chromosomes must also make copies of themselves (replicate) to maintain the appropriate chromosome number in the cells.
- In eukaryotes, the chromosomes replicate in two main types of nuclear divisions, called mitosis and meiosis.
- These two types of divisions are quite different and have different functions, some of the molecular features are held in common like- DNA replication, adhesion of replicated chromosomes, movement of chromosomes into cells.
- Mitosis is the nuclear division associated with the asexual division of cells.
- In multicellular organisms mitosis takes place during the division of somatic cells, the cells of the body.
- In single-celled eukaryotes such as yeasts, mitosis occurs in the cell divisions that cause population growth.
- Since asexual cell division is aimed at straightforward reproduction of cell type, it is necessary for the set of chromosomes to be maintained constant through the cell, and this is what mitosis achieves.
The stages of the cell division cycle are similar in most organisms. The two basic parts of the cycle are interphase (comprising gap 1, synthesis, and gap 2) and mitosis.
An event essential for the propagation of genotype takes place in the S phase (synthesis phase) because it is here that the actual replication of the DNA of each chromosome occurs.
As a result of DNA replication, each chromosome becomes two side-by-side units called sister chromatids. The sister chromatids stay attached through the action of specific adherence proteins.
Stages of mitosis
- Prophase: The pairs of sister chromatids, which cannot be seen during interphase, become visible. The chromosomes contract into a shorter, thicker shape that is more easily moved around.
- Metaphase: The sister chromatid pairs come to lie in the equatorial plane of the cell.
- Anaphase: The sister chromatids are pulled to opposite ends of the cell by microtubules that attach to the centromeres. The microtubules are part of the nuclear spindle, a set of parallel fibers running from one pole of the cell to the other.
Nuclear spindle fibers provide the motive force that pulls apart the chromosomes or chromatids in mitosis and meiosis. These spindle fibers are polymers of a protein called tubulin.
Each centromere acts as a site to which a multiprotein complex called the kinetochore binds. The kinetochore acts as the site for attachment to spindle fiber microtubules.
- Telophase: Chromatids have arrived at the poles and the pulling-apart process is complete. A nuclear membrane reforms around each nucleus, and the cell divides into two daughter cells. Each daughter cell inherits one of each pair of sister chromatids, which now become chromosomes.
Thus overall, the main events of mitosis are replication and sister chromatid adhesion, followed by segregation of the sister chromatids into each daughter cell. In a diploid cell, for any chromosomal type the number of copies goes from 2 → 4 → 2.