When the cell loses the power to control its multiplication and the cell cycle results in excessive cell formation by replication method. This condition is called 'cancer'. The science which deals with cancer is called 'Oncology'.
Formation of cancer cells: The first step for formation of cancer is initiation of the cancer cell production. For this step 'mutation' plays a vital role. The mutated cell escapes from the cell cycle and undergo proliferation of growth and then to division. It keeps on dividing to form the cell mass. Thus the first set of cells to initiate further growth attains. Soon these cells keep on proliferating to produce a visible mass.
The mutation of cells are produced by both the influence of environmental and the chemical factors. It is also believed that the mutated genes present in the parents are transmitted to the off springs. The parent having the mutated cell wont express the disease although they carry it.
The genes which are mutated to form cancer are called "Oncogenes".
The oncogenes may be in the following form:
1. Viral oncogenes: It causes minority of cancers. The DNA sequences are introduced exogenous form through viral infection. Also it is believed that our ancestors been attracted with viral infection and the infected genes are transmitted to the next generation.
Good example for this is Human T-Cell Lymphotrophic virus type- 1 which causes Leukemia
Other cancers associated with the viral infections are Human Immunodeficiency virus (HIV), Epstein- Barr virus (EBV), Herpes simplex virus 2(HSV2)
There is a mutation in the components which are essential for signaling transduction pathway. Hence they produce increase or altered copies of genes which are responsible to control a particular functions in the body.
Examples for the cancers produced by this genes are H-RAS oncogene is the causative agent for the removal of GTP which would mimic like Growth hormone stimulation.
In Chronic Myeloid Leukemia, the trans location of chromosome 9 to chromosome 22 leads to the excessive proliferation of the while blood cells.
3.Tumor suppressor genes:
Certain genes in our body have the code proteins for normal functions, which helps for the regular cell cycle and the death of the cell at time. The genes which responsible for this action altered resulting in tumors.
Retinoblastoma is well known example for it. Neurofibromin encodes RAS Inhibitor which is mutated in Neurofibromatosis.