49. Normal Cell Growth: The Cell Cycle
The cell cycle is a critical process that a cell undergoes in order to copy itself exactly. Most cancers have mutations in the signals that regulate the cell's cycle of growth and division. Normal cell division is required for the generation of new cells during development and for the replacement of old cells as they die.
Most cells remain in interphase, the period between cell divisions, for at least 90 percent of the cell cycle. The first part of the interphase is called G1 (for first gap), followed by the S phase (for DNA synthesis), then G2 (for second gap). During G1, there is rapid growth and metabolic activity, including synthesis of RNA and proteins. Cell growth continues during the S phase, and DNA is replicated. In G2, the cell continues to grow and prepares for cell division. Cell division (mitosis) is referred to as the M phase. Cells that do not divide for long periods do not replicate their DNA and are considered to be in G0.
In normal cells, tumor suppressor genes act as braking signals during G1 to stop or slow the cell cycle before S phase. DNA repair genes are active throughout the cell cycle, particularly during G2 after DNA replication and before the chromosomes prepare for mitosis.