Cell cycle
In recent years, the way in which cell division is controlled has been clarified. Inside each cell there is a cell cycle ‘clock’ which determines whether or not a cell should divide. The clock is an executive decision maker and integrates the regulatory signals received by the cell with the current state of health of the cell. The cell cycle consists of four steps. In the gap 1 (G1) phase, the cell grows in size and checks the status of its internal systems. If everything is functioning normally, and any damage to the DNA has been corrected, the cell moves on through the cycle. If something is wrong and cannot be corrected, the cell halts its progression through the cycle and may initiate apoptosis and close down. R marks the point where restriction of the cycle can occur.
In the following synthetic (S) phase, the cell replicates its store of DNA in the chromosomes. Following this there is a period of preparation for division called the G2 phase. Then the cell divides - the mitotic (M) phase. The two new daughter cells then enter the G1 phase of their own cell cycle.
This sequence of events involves interactions between many different proteins, some of which are capable of halting the process if conditions are unfavourable. p53 is a tumour suppressor protein that binds to specific DNA sequences. It is thought of as the "guardian of the genome" and controls the cell cycle to enable the repair of damaged DNA.
The control of cell proliferation is intimately connected to apoptosis - a process by which cells methodically close down their metabolic activities and die when they have irreparable damage to their DNA or have no further role in the body. Normal p53 suppresses tumour growth by arresting cells in G1 phase or triggering apoptosis. The p53 gene is mutated in a wide range of tumours, for example: skin cancer and colorectal cancer, with the result that in the affected cells the cell cycle clock spins out of control and the cells divide without restraint.