by Sarah Downie

Created on: January 28, 2007   Last Updated: May 02, 2007

Cells need to divide and to do this they follow the process of mitosis. Mitosis produces genetically identical daughter cells. Organisms also need to have cell division to form many different genotypes. Cells are organised into tissues, which are organised into organs.
Mitosis is broken up into four sub-phases that can be distinguished by using a light microscope. Each stage progresses to the next as a continual process. The four stages are: prophase, metaphase, anaphase and telophase.

Interphase is sometimes known as the "resting phase", however in biochemical terms it is a very active period of growth. Interphase can be split into three parts: the period of DNA replication is called the S phase, Gap phase G1 and Gap phase G2.
At the end of interphase, cells enter prophase. The nucleolus disappears and the chromatin starts to condense into discrete chromosomes. Each duplicated chromosome appears as two identical sister chromatids joined at the centromere. In the cytoplasm, the mitotic spindle begins to form between the two centrosomes. This process pushes the two centrosomes away from each other along the surface of the nucleus.
In the initial stage of metaphase (prometaphase), the nuclear envelope breaks up. Bundles of microtubules extend from each pole towards the equator of the cell and attach to the centromere of each chromatid. As the cell enters metaphase the chromosomes are now at opposite poles of the cell. The chromosomes line up on the metaphase plate in the middle of the cell. At the end of metaphase, the cells enter anaphase.
In anaphase, the paired centromeres of each chromosome separate and the chromatids begin moving apart as the spindle shortens. The poles also move further apart. Each chromatid can now be called a chromosome. By the end of anaphase, the two poles of the cell have an identical and complete set of chromosomes. At the end of anaphase, cells enter telophase.
The cell lengthens during telophase and nuclear envelopes form around each set of chromosomes. Nucleoli reappear and the chromosomes start to uncoil. Cytokinesis also takes place during this period, which in animal cells involves the formation of a cleavage furrow that pinches the cytoplasm into two, resulting in the appearance of two separate daughter cells. The presence of a cell wall makes cytokinesis quite different in plant cells. There is no cleavage furrow. Instead, a cell plate is formed between the two daughter cells.

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