Cellular Biology

The Stages of a Mitotic Cell Cycle



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Mitosis is the process by which eukarotic cells divide, the two daughter cells produced are identical (with a few mutations): in addition to this there is another important difference, that is that the cells produced are diploid (2n) rather than haploid (n) as is the case with cells after meiosis, this is important as in reproduction (involving the fusion of two cells produced by meiosis) the 2n number is restored with one copy of each gene from the ovum and one from the sperm.

Mitosis involves several distinct steps that take place sequentially and lead from one cell dividing into two, these steps are as follows:

(i) Interphase - this involves three smaller stages; however for simplicity I will discuss only that which occurs generally in the interphase, which is the replication of all the cell genomic information (in the form of two identical sister chromatids for each chromosome that are joined at a centromere), organelles as well as cell growth such that when cell division occurs each cell is not significantly smaller than the original.

(ii) Prophase - the stage where the chromatin of the nucleus condense to form visible chromosome structures.

(iii) Metaphase - the nucleus has broken down and hence the chromosomes are allowed to line up along the centre of the cell with the chromosomes lined up such that one of each identical sister chromatid is on each side whilst still remaining attached at the centromere.

(iv) Anaphase - the chromatids are pulled appart by spindle fibres so that one of each pair is pulled to opposite poles.

(v) Telophase - the nuclear membrane reforms around each set of DNA. This occurs at the same time as cytokinesis as the cells separate from each other.

In addition to this cell divion can be unequal, as if the chromosomes in the metaphase line up to one side the cells produced will be of different sizes. In addition to this it can be seen that there can be directionality to a mitotic divison, which is important in development as this allows a developing fetus to grow in certain directions and hence not be shapeless.

It is easy to see how mitosis is important in biology as if it were not to work correctly cells would not reproduce in a controllable manner and hence cell division would result in organisms having too much or too little DNA, which is distasterous in most instances. It is also necessary to control the rate of replication as a cell with mutations such that it replicates in an uncontrollable manner is a tumour cell and so one can see, by looking at cancer how important controlled replication is.

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