Question

imagine that there is a mutation in a cdk gene such that its gene product is present but nonfunctional. based on what you know about cdk and protein rb interactions, what kind of effect would this mutation have on a mammalian cell that has received a growth signal? a. the cell would enter the cell cycle but not be able to undergo cytokinesis. b. the cell would remain in g1. c. the cell would skip s phase and enter g2. d. the cell would replicate its dna and then fail to enter g2. e. the cell would not be able to phosphorylate its associated cyclin

Answer 1

The cell would remain in G1. Hence the correct option is b.

The cyclin-dependent kinase (CDK) plays a crucial role in cell cycle regulation, particularly in the transition from G1 to S phase. When a growth signal is received, CDK forms a complex with its regulatory partner cyclin, leading to activation. This active CDK-cyclin complex phosphorylates the retinoblastoma protein (pRB), causing its inactivation. In its active form, pRB restricts the progression of the cell cycle by inhibiting the activity of transcription factors necessary for the expression of genes required for DNA synthesis.

If a mutation occurs in the CDK gene, rendering its gene product nonfunctional, the CDK-cyclin complex cannot be properly activated. Consequently, the downstream targets like pRB remain active and inhibit the transition from G1 to S phase. This results in the cell remaining in the G1 phase, unable to progress into the synthesis (S) phase of the cell cycle. Therefore, the correct answer is (b) – the cell would remain in G1. The mutation disrupts the normal cell cycle progression, highlighting the essential role of functional CDK in the regulation of cell cycle transitions in response to growth signals.