Mitosis

Mitosis is the process of nuclear division that produces two identical daughter cells with a diploid complement of chromosomes from one also identical parent cell. Check it out on The Mitosis Cycle on YouTube. It can be broken down into four main stages seen in animal cells:

Interphase

If a cell is not undergoing mitotic cell division, the cell is in interphase. Interphase constitutes for most of the cell cycle;

It is the preparation period preceding mitosis, in which the cell grows and it's DNA is replicated. Interphase can be divided into 3 main phases;

G1( gap phase 1 ), S ( synthesis phase ) and G2 ( gap phase 2).

During Gap phase 1 biosynthetic activity in the cell is increased as the cell grows and replicates it's organelles whilst producing spare proteins.

During S phase the cell must replicate it's DNA, in order for the two daughter cells produced via the following division to have a complete set of genetic material. Chromosomes are duplicated during this same phase, this produces 2 sister chromatids, allowing each daughter cell which results from the mitotic division to have the diploid number of chromosomes.

During Gap phase 2 the cell continues to grow and it produces specific proteins necassary for cell division so that mitosis can follow interpahse.

Another vital event which occurs during interphase is that the cell repairs any DNA dammage, to ensure that no undesirable mutations are passed on to the progeny cells.

Prophase

Chromosomes condense (shorten and thicken) to form visibly distinct threads within the nucleus. This is brought about by the action of large proteins called condensins. Each chromosome at this point consists of two chromatids held together at the centromere (central point). Finally nucleoli disappear and the nuclear envelope disintegrates.

The four phases of mitosis.

Metaphase

The mitotic spindle consisting of microtubules formed by the polymerisation of tubulin begins to form. There are three types if microtubule in the spindle:

1. Anchor (centrosome to cell membrane)
2. Arch (between centrosomes)
3. Attachment ( to chromosomes)

NB. Centrosomes are located at the ends/poles of the spindle and are microtubule organising centres where tubulin polymerisation is initiated.

The next step in metaphase is the formation of the kinetochore, a site of attachment between the chromosome and the spindle microtubules . Only chromosomal microtubules that make contact with the kinetochore however, will become stabilised and others will depolymerise.

The final part involves a position near to the centre of the cell, known as the metaphase plate. The kinetochore signals for each chromosome to move to the plate via dephosphorylation of certain proteins. The cell can then only move on to anaphase when all kinetochores are under tension and are aligned on the metaphase plate.

Anaphase

Proteins holding the chromatids together dissolve and centromeres separate (This is also known as disjunction). Spindle fibres are signalled by the cell to lengthen causing the identical sister chromatids of each chromosome to be pulled toward opposite poles of the spindle . The two groups that now lie in each end of the spindle contain the same number of chromosomes that were present in the original interphase nucleus.

Above shows the four stages of mitosis .

Telophase

In this final stage of mitosis, a nuclear envelope forms around each group of chromosomes and cytokinesis occurs, where the cytoplasm divides into two. The spindle completely disappears and chromosomes undergo the reversal of condensation, deeming them no longer visible under the light microscope. At the end of telophase the two separate daughter cells assume interphase appearance with fully developed nucleoli.