During a normal cell growth, gene mutations occur. Cells generally recognize a mistake and repair it. However, when this mistake is missed, the cells become cancerous.

Gene mutations accelerate rapid growth and division of cells which results in birth of new cells that have same mutations. Normal cells stop growing after a certain point of time but the cells that become cancerous lose that trait and continue growing rapidly. Cancer is caused when the mutation causes the DNA within the cells to change. This leads to the cell stopping from performing its normal functions and makes the cell cancerous.