Final answer:
The correct answer to the student's question is E. maturation promoting factor (MPF), as it is the key regulator for the cell to progress from the G2 phase into mitosis.
Step-by-step explanation:
The progression from G2 to M, the G2/M transition, is a crucial phase in the cell cycle. This transition is regulated by checks and balances that ensure cells divide correctly without mutations that could lead to cancer. Here, we will explore the entities responsible for the control of this important checkpoint.
The checkpoint mechanism at the G2/M transition involves the activation of various proteins that confirm the cell is ready to proceed to mitosis. Among these proteins, the maturation promoting factor (MPF), a complex of cyclin B and Cdk1, plays a key role. As cells prepare for mitosis, cyclin B accumulates and assists in the activation of Cdk1, forming the active MPF. This is required for the cell to progress from the G2 phase into mitosis.
In this context, p53 and p21 chiefly act at the G1 checkpoint and are responsible for halting the cell cycle in the presence of DNA damage by binding to and inhibiting the Cdk/cyclin complexes. While overexpression of cyclin D, DNA polymerase processivity, and the Bax tumor-suppressor protein have roles in cell cycle regulation, they are not directly responsible for controlling the G2/M transition. Therefore, the correct response to the student's question is E. maturation promoting factor, which directly influences the G2/M transition.