CASE 2 WHEN GOOD CELLS GO BAD

As we have just seen, cells have evolved mechanisms that promote passage through the cell cycle, such as the cyclin–CDK complexes, as well as mechanisms that halt the cell cycle when the cell is not ready to divide, such as the DNA damage checkpoint that depends on the p53 protein. When cellular mechanisms that promote cell division are inappropriately activated or the normal checks on cell division are lost, cells may divide uncontrollably. The result can be cancer, a group of diseases characterized by rapid, uncontrolled cell division.

In this section, we explore various ways in which cell cycle control can fail and lead to cancer. We begin with a discussion of a cancer caused by a virus. Although most cancers are not caused by viruses, the study of cancers caused by viruses helped us to understand how cancers develop.