1. Immortalization of cancer cell: Cancer cells are characterized by immortalization; they overcome apoptosis by altering the genes that control such death of the cell. Genetic expressions of certain genes make the malignant cells immortal. These two genes are oncogenes and tumor suppressor gene. Actually activation of protooncogene and inactivation of tumor suppressor genes both are responsible for the progression of cancer.
2. Heterozygous oncogenic mutation, homozygous tumor supression mutation in cancer cell. Oncogene mutation is dominant and thus if one allele of the oncogene in the chromosome is mutated the cell will become cancerous. For example, if c-ras in one chromosome undergoes mutation into v-ras, in heterozygous condition it induces cancer.
On the other hand, tumor suppressor gene mutation is recessive; therefore mutations in both alleles on the two chromosomes are essential in homozygous normal individuals. Thus Rb (retinoblastoma) gene mutations in both chromosomes cause cancer.
3. Cancer cells are genetically unstable: The majority of human cancers show signs of dramatically enhanced mutation rate, the cells are said to be genetically unstable. This instability can take various forms. Some cancer cells are defective in the ability to repair local DNA damage to correct replication error. Other cancer cells have trouble for maintaining the integrity of their chromosomes and thus display abnormalities in their karyotype. The inability of mismatch replication is due to the mutation of p53 protein gene, which is called guardian of the genome. Normally, the p53 protein repairs replication error and prevents genetic instability. Loss of this gene in both homologous chromosomes causing the cells to proceed towards cancer.