Question

Rb is a negative regulator that blocks the cell cycle at the G1 checkpoint until the cell achieves a requisite size. What molecular mechanism does Rb employ to halt the cell cycle?

a) Rb inhibits cyclin synthesis
b) Rb activates cyclin-dependent kinases
c) Rb prevents cyclin binding to Cdk
d) Rb induces DNA replication

Answer 1

Rb inhibits cell cycle progression at the G1 checkpoint by binding to and inactivating the transcription factor E2F, preventing the transcription of genes needed for DNA synthesis and cell cycle advancement.

Step-by-step explanation:

The retinoblastoma protein (Rb) functions as a negative regulator of the cell cycle, particularly at the G1 checkpoint. In its active, dephosphorylated state, Rb binds to transcription factors, primarily E2F. This binding prevents E2F from activating genes necessary for the transition from G1 to the S phase of the cell cycle. Consequently, proteins required for DNA synthesis are not produced. As cells grow and increase in size, Rb becomes phosphorylated, which inactivates it, leading to the release of E2F. E2F can then stimulate the expression of genes that promote the cell cycle progression. Thus, the molecular mechanism employed by Rb to halt the cell cycle involves the inhibition of transcription factors responsible for advancing the cell through the cycle.