Answer:
Telomeres
Step-by-step explanation:
Telomeres are structures at the ends of chromosomes that protect the DNA. They get shorter as we age. When they become too short, the cell undergoes apoptosis (programmed cell death ) to avoid having cells with DNA that could become damaged.
However, cancer cells can sometimes overcome this, and can avoid telomere shortening, or avoid apoptosis once telomeres have become too short. This allows cancer cells to divide indefiniately.