Cell division; stimulation and suppression

Healthy body tissue cells divide by mitosis so that growth and repair can take place. The regulatory mechanism involves specific genes that carefully coontrol the process. In particular, normal cellular genes, called proto-oncogenes, code for proteins that stimulate normal cell growth and division. When growth or repair is completed, other genes called tumour supressor genes inhibit cell divsion
The proteins encoded by tumour suppressor genes may; repair damaged DNA before it can be replicated, control cell adhesion and ensure that cells are anchored in their proper place, inhibit cell division
In a healthy cell, the activites of these proto-oncogenes and tumour supressor genes are in balance. Problems arise when the genes mature or other control mechanisms break down so that cells divide uncontrollably
Proto-oncogenes can mutate into cancer-causing genes called oncogenes. The presence of oncogenes leads to either; an increase in the amount of proteins that stimulate cell growth and division. OR.. an increase in the activity of each growth-stimulating protein molecule- the mutation may lead to an altered protein that is hyperactive or that resists being broken down when it is no longer needed
Either of these effects can lead to abnormal stimulation of the cell cycle and result in the development of a malignant tumour.
Any mutation that decreases the activity of tumour supressor genes can also lead to a malignacy. Such a mutation mat allow the rate of cell division to increase through the lack of suppression, or it may lead to cells losing thrit normal anchorage and invading other parts of the body where they might develop into a malignant tumour
The mutation of either a proto-oncogene into an oncogene or a tumour suppressor gene into one that no longer suppresses cell division may be spontaneous, or it may be triggered by specific cancer-causing agents called carcinogens. Spontaneous muations may occur as random events during DNA replication. They may lead to a point mutation that alters the nucleotide base sequence in the gene and the protein encoded by it
Factors that trigger muations include; chemical carcingoens, such as those in tobacco smoke; physical agents, such as X-rays; and certian viruses.
Retroviruses capsules contain RNA and reverse transcriptase,an enzyme that transcribes DNA from the template formed by the RNA. The resulting viral DNA, which may contain an oncogene, is incorporated into a hosts cell as a provirus. The hosts RNA polymerase transcribes the virla DNA into RNA molecules. This RNA may finction as mRNA to make viral proteins, or it may form the RNA contained within the capsule of new viruses particles released from the cells
Most mutated cells are either destroyed by the body's immune system or die, causing no harm. However, a single mutated cell divide to form a clone of idnetical cells. Eventually a mass of abnormal cells called a tumour is formed. Benign tumours such as warts do not spread form their point of origin. Tumours that can spread through the body are called maligant tumours

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Cell division; stimulation and suppression

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