Which of the following events is most closely associated with the Start of the c

Question

Which of the following events is most closely associated with the Start of the cell cycle? O The Cdc25 phosphatase removes an inhibitory phosphate from M-Cdk. O Tension sensing mechanisms on the spindle trigger the anaphase promoting complex (APC) once all the chromosomes are attached by their kinetochores. O The Retinoblastoma (Rb) protein gets phosphorylated, causing it to no longer inhibit transcription of the S-cyclin gene. Activated p53 promotes transcription of the p21 gene which triggers a checkpoint.

Explanation / Answer

C. During the early G1 phase, the transcriptional repressor Rb(retinoblastoma) protein binds to transcriptional factors in order to prevent G1 - S transcription. This represses transcription of G1 - S promoting proteins.

Upon the decision of cell to progress past the G1 check point, phosphorylation of Rb occurs which leads no longer inhibition of transcription of S cyclin gene.

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