Figure 37. The cell cycle and RB.

The cell cycle is depicted, showing the phases (divided by chevrons): growth or gap 1 (G₁), DNA synthesis (S), growth or gap 2 (G₂) and mitosis (M) in a circle, with exit from cycle represented as G₀. Cell cycle check points are depicted as bars, G₁/S check point (red: a DNA damage check), S-phase check point (blue: a DNA damage and replication fork check), G₂/M check point (green: a DNA damage and completion of replication check) and spindle check point (yellow: ensuring correct alignment of the chromosomes upon the spindle, ready for division). The cyclin and CDK complexes relevant to each phase are shown. Central to cell cycle control is the TSG RB. RB becomes increasingly phosphorylated by the activated CDKs through G₁ (as depicted by increasing P). As the cycle progresses, it becomes hyperphosphorylated and this allows entry into S phase and further progression. Un-phosphorylated RB blocks cell cycle progression. During G₁, the cycle can be initiated via mitogenic signalling (purple arrows). Once past ‘start’ the cell is committed to cycle.