Cell division

Before dividing, a cell must copy its entire genome, and every copy introduces a small chance of error, so tissues that divide often (gut lining, blood, skin) accumulate mutations faster and give rise to more cancers. Classic chemotherapy targets division itself, damaging DNA or the machinery that pulls chromosomes apart, which is why it also hits hair follicles, bone marrow and gut lining and produces the familiar side effects. The number of divisions a tumour has undergone shows up as the number of mutations it carries, and the rate of division (Ki-67 staining, mitotic count) feeds into tumour grade.