A point mutation is also called a single base substitution. A replacement of a base nucleotide with another one occurs. It can also mean that a single base is inserted or deleted. This type of mutation can occur spontaneously during DNA replication. It causes the DNA sequence to change and therefore the amino acid sequence in a protein may change resulting in a defective and possibly non-functional protein. Mutagenic agents can increase the chance of this occurring including X-Rays, U.V. radiation, chemicals, heat, environmental changes. If a mutation occurs in a key regulatory gene such as a tumor suppressor gene or a proto-oncogene, unregulated cell division can occur, resulting in cancer. These mutations can usually be repaired by DNA repair mechanisms, however, over time and as people age, the exposure to mutagenic agents and changes that could accumulate in a cell's DNA, could lead to a greater likelihood of cancer occurring in an individual.
when the cells of body divides to form an abnormal mass of cells called a tomor which invades and destroy neighbouring tissue but all tumors are not cancerious.two major classes of genes contribute to causing cancer.these are oncogenes and tumor supressing genes.oncogenes must be activated and tumor supressor gene must be inactivated to initiate cancer.
when X-Rays passes through human body in few cases break down the sequence of DNA this breakage of sequence leads to cancer.