Which sequence best describes apoptosis at the G1 checkpoint?

The correct answer is "Cdk/cyclin complex is inhibited→ p53 levels rise → p21 production increase".

Explanation:

The missing options of this question are:

A. p53 levels rise → p21 production increase → Cdk/cyclin complex is inhibited

B. p21 production increase → Cdk/cyclin complex is inhibited→ p53 levels rise

C. Cdk/cyclin complex is inhibited→ p21 production increase → p53 levels rise

D. Cdk/cyclin complex is inhibited→ p53 levels rise → p21 production increase

E. p53 levels rise → Cdk/cyclin complex is inhibited→ p21 production increase

The correct answer is option D. "Cdk/cyclin complex is inhibited→ p53 levels rise → p21 production increase".

Apoptosis is a process at which the cells undergo a programmed death. This mechanisms happens at G1 checkpoint first by the inhibition of Cdk/cyclin complex which causes p53 levels to rise and later, an increment of p21 production. First, a cyclin-dependent kinase inhibitor regulates the inhibition of the Cdk/cyclin complex. This causes p53 levels to rise, since p53 is a direct downstream kinase substrate for CDK. Subsequently, p21 production increase as well since p21 is a inhibitor of cyclin-dependent kinases.

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