Which checkpoint ensures the spindle is formed and organelles needed for mitosis exist?

• A. G1 checkpoint
• B. G2 checkpoint
• C. M checkpoint
• D. S checkpoint

Answer: (C)

Cell cycle checkpoints verify that key events are finished before the cell moves on. The checkpoint that watches the formation of the spindle and the proper attachment of chromosomes to it is the spindle assembly checkpoint. It operates during metaphase and delays progression to anaphase until every chromosome is properly bi-oriented and under the right tension across its sister chromatids. This delay ensures the spindle is fully formed and capable of pulling sister chromatids apart, so mitosis proceeds only when the equipment and attachments are correct. If the spindle isn’t ready or attachments are off, the checkpoint halts the cycle to prevent mis-segregation. Other checkpoints monitor different stages: G1 checks growth conditions and DNA integrity before replication; G2 checks that DNA has been replicated and that the cell is ready for mitosis (including organelle readiness); S checks ongoing DNA replication for completion and fidelity. The description given aligns with the spindle assembly checkpoint’s role.