140. Which one of the following is NOT a major checkpoint in the cell division cycle of eukaryotic
cell?
A. Beginning of S-phase
B. Transition from S to G2 phase
C. Transition from G2 to M phase
D. Transition from Metaphase to Anaphase during Mitosis

Major Checkpoints in the Eukaryotic Cell Cycle

The cell cycle is a highly regulated process that allows cells to grow, replicate their DNA, and divide into two daughter cells. Proper regulation of the cell cycle is crucial for the maintenance of healthy cellular function and prevents uncontrolled cell division, which can lead to diseases like cancer.

In eukaryotic cells, there are several key checkpoints during the cell cycle that ensure that each phase is completed correctly before the next phase begins. These checkpoints monitor different aspects of cell division, such as DNA integrity, proper chromosome segregation, and sufficient cell growth. Let’s explore the major checkpoints and their role in regulating the cell cycle.

Major Checkpoints in the Eukaryotic Cell Cycle:

1. Beginning of S-phase:

   • Purpose: This checkpoint ensures that the cell has grown sufficiently and that all the necessary proteins and machinery for DNA replication are available.

   • Key Regulation: The restriction point (also called the G1/S checkpoint) occurs right before the S-phase, where the cell decides whether to continue with division or enter a resting phase.

2. Transition from S to G2 phase:

   • Purpose: This checkpoint verifies that DNA replication has been completed successfully and that there are no DNA errors before the cell enters the G2 phase, which prepares the cell for mitosis.

   • Key Regulation: This checkpoint ensures that all the DNA is properly replicated without errors or damage.

3. Transition from G2 to M phase:

   • Purpose: This checkpoint ensures that the cell is ready to enter mitosis. It checks for DNA damage and ensures that the cell has sufficient resources to complete cell division successfully.

   • Key Regulation: The transition is regulated by cyclin-dependent kinases (CDKs) and ensures that the cell has properly prepared for mitosis.

4. Transition from Metaphase to Anaphase during Mitosis:

   • Purpose: This is a critical checkpoint in the mitotic phase that ensures chromosomes are properly aligned on the spindle before anaphase begins. The cell will not proceed to anaphase until all chromosomes are attached to the spindle fibers and aligned at the metaphase plate.

   • Key Regulation: The spindle assembly checkpoint (SAC) monitors chromosome alignment and ensures that the chromatids are ready to be pulled apart evenly.

The Correct Answer

The correct option for the checkpoint that is NOT a major checkpoint in the eukaryotic cell cycle is:

Answer: D. Transition from Metaphase to Anaphase during Mitosis

This transition occurs during the mitotic phase, and although it is a critical step in the cell cycle, it is part of the broader M-phase (mitosis) rather than a specific, pre-mitotic checkpoint like the other options listed.

Conclusion

The eukaryotic cell cycle is carefully regulated by a series of checkpoints to ensure accurate DNA replication, chromosome segregation, and proper division. While the transition from Metaphase to Anaphase is crucial during mitosis, it is not typically categorized as one of the major checkpoints that govern the cell cycle’s overall regulation before entering mitosis. Understanding these checkpoints helps provide insight into cellular processes and diseases linked to cell division errors.