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Discussion

The retinoblastoma protein, Rb, restrains the cycle by

  • A.cutting the DNA molecule at every origin of replication
  • B.degrading all of the cyclins as fast as they accumulate
  • C.attaching each of the chromosomes to the mitotic spindle
  • D.binding and inhibiting the E2F transcription factor

Answer: D

Phosphorylation by a cyclin-CDK complex releases E2F, which then switches on the genes needed for S phase. Loss of Rb removes that restraint and the cell enters S phase regardless. Rb was the first tumour suppressor to be identified.

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