3.8 The cell cycle

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Cell cycle overview

Only some cells in multicellular organisms retain the ability tom divide. Those that do, do not divide continuously, but undergo a regular cycle of division separated by periods of cell growth. This is known as the cell cycle and has three stages:

1.       Interphase which occupies most of the cell cycle, and is sometimes known as the resting phase because no division takes place.

2.       Nuclear division when the nucleus divides either into two (mitosis) or four (meiosis).

3.       Division of the cytoplasm (cytokinesis) which follows nuclear division and is the process by which the cytoplasm divides to produce two new cells (mitosis) or four new cells (meiosis).

The length of a complete cell cycle varies greatly amongst organisms. Typically, a mammalian cell takes about 24 hours to complete a cell cycle, of which about 90% is interphase.

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Cancer and the control of mitosis

Cancer is a group of diseases caused by a growth disorder of cells. It is the result of damage to the genes that regulate mitosis and the cell cycle. This leads to uncontrollable growth and division of cells. As a consequence, a group of abnormal cells, called a tumour, develops and constantly expands in size. Tumours can develop in any organ in the body, they are most commonly found in the lungs, prostate gland, breast and ovaries, large intestine, oesophagus and pancreas. A tumour becomes cancerous when it changes from being benign to malignant.

Most cells divide by mitosis for either growth or repair. The rate of mitosis can be affected by the environment of the cell and by growth factors. It is also controlled by two types of gene, a mutation of one of these results in uncontrolled mitosis. Most mutated cells die, however, any that survive are capable of forming clones of themselves and forming tumours. Malignant tumours grow rapidly, are less compact and more likely to be life-threatening. Benign ones grow more slowly, are more compact and less likely to be life-threatening.

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Treatment of cancer

The treatment of cancer often involves killing dividing cells by blocking a part of the cell cycle. The cell cycle is disrupted and cell division, hence cancer growth, ceases. Drugs used to treat cancer (chemotherapy) usually disrupt the cell cycle by:

-          Preventing DNA from replicating

-          Inhibiting the metaphase stage of mitosis by interfering with spindle formation

The problem with these drugs is that they also disrupt the cell cycle of normal cells. However, the drugs are more effectively against rapidly dividing cells and because cancer cells divide more rapidly they are more greatly effected by the treatment. Normal body cells that also divide at a faster rate than normal, like hair producing cells, are more vulnerable to damage.

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