Cancer has become the most fatal disorder these days. Genetically Cancer can be looked upon as a problem with differentiation and development. When normal cells of animals differentiate, there are control mechanisms to regulate their growth and division. When cells multiply, they become crowded and start touching each other. This surface contact results in inhibition of cell movement and division.
On the other hand, cells which continue to divide and give rise to a tissue mass referred to as tumour. These can also invade other tissues and tumours can arises in new locations, this is cancer. The genetic information is present in codon form and due to the attack of X-rays, viruses and chemical mutagens these codons get change. Epstein-barr, herpes virus and Papilloma virus are main cancer causing viruses. Many cancers appear essentially due to the activation of a set of genes referred to as ONCOGENES.
There are about 20 viral genes, which are known to cause cancer. These genes are called VIRAL ONCOGENES. Surprisingly, each VIRAL ONCOGENES has a homologous base sequence in the DNA of the normal animal cells. This sequence is named PROTOONCOGENE. PROTOONCOGENES control the production of proteins, which stimulate the growth and cell division. PROTOONCOGENES turned on to cause cancer and this happens in 2 ways -
- A PROTOONCOGENE is placed next to a gene that is not normally next to it and this boosts its expression.
- PROTOONCOGENE is moved from its normal position on a chromosome and placed next to a gene, which is normally very active.
Many PROTOONCOGENES are involved in normal growth and development of cells. In cancer this normal function is subverted so that the PROTOONCOGENES are slightly altered or broken into fragments or amplified and are transferred to a new chromosomal location. These ONCOGENES now come under an altered environment and different regulatory controls. All these changes result in altered genomic complement and lead to the formation of abnormal proteins, which now are responsible for the uncontrolled growth, which is cancerous.