Overview
When Growth Control Is Lost
Cancer results from uncontrolled cell proliferation caused by accumulated mutations in genes regulating cell growth, differentiation, and apoptosis.
Cancer results from uncontrolled cell proliferation caused by accumulated mutations in genes regulating cell growth, differentiation, and apoptosis. A normal cell can pause, repair damage, enter a resting state, or die; a malignant cell progressively loses those restraints and passes its abnormal behaviour to its descendants. The cell cycle consists of G1, when the cell grows and prepares for DNA synthesis; S phase, when DNA is replicated; G2, when the cell checks its preparation for division; and M phase, when mitosis and cytokinesis occur. G0 is a relatively quiescent state. Control points normally prevent a cell with serious DNA damage from progressing through this sequence. Proto-oncogenes normally promote appropriate growth and survival. When activated or amplified, they become oncogenes and send growth signals when the cell should be resting. Tumour-suppressor genes normally slow proliferation, repair DNA, or trigger apoptosis; their inactivation removes those brakes. Cancer usually reflects several cooperating changes rather than one isolated mutation, allowing a clone to survive, expand, recruit blood supply, invade tissue, and sometimes spread through lymphatic or blood vessels. This biology explains why treatment can...