Cell cycle regulation. Animation showing the role cyclin D plays in the regulation of the cell cycle. The cell cycle consists of four phases. During the S phase DNA (deoxyribonucleic acid) is replicated in preparation for nuclear division. This is followed by the G2 phase, during which the cell continues to grow. Next is the M phase, when the cell undergoes mitosis (nuclear division) to produce two daughter cells. The daughter cells enter the G1 phase, during which they increase in size and produce the proteins that will be needed in the S phase. There are a number of checkpoints in the cell cycle to ensure that damaged cells are not allowed to proliferate. Progression from the G1 phase to S phase is initiated by the protein cyclin D (yellow dot), which is synthesised in response to growth factors. Cyclin D binds to the enzyme Cdk4 (purple), activating it and beginning a sequence of reactions that lead to the S phase. Once the cell has entered the S phase the cyclin D is degraded.
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