Biology · Cell Cycle and Cell Division · NEET
The G1/S checkpoint (the DNA-integrity or replication checkpoint). A stalled replication fork means DNA copying has a problem, so the cell must not move ahead. This checkpoint guards the entry from G1 into S phase and halts the cycle before the damage spreads. The G2/M and M checkpoints act later and are not the main responders to a stalled fork. This was tested directly in NEET 2016.
They check different things at different times. G1/S checks whether the cell is big enough and its DNA is undamaged before starting DNA copying (S phase). G2/M checks that DNA copying is fully finished and error-free before the cell enters mitosis. The M checkpoint (spindle checkpoint) checks that every chromosome is properly attached to the spindle fibres before anaphase begins. Simple order to remember: check before copy, check after copy, check before splitting.
APC is a protein-degradation machine. When the M (spindle) checkpoint is satisfied, APC tags and destroys the proteins that hold sister chromatids together at the centromere. Once this glue is gone, the centromere can split and the two sister chromatids move to opposite poles. So APC is the trigger that starts anaphase in mitosis.
Chromosomes will not segregate. Because APC destroys the proteins that hold sister chromatids together, a defective APC means that glue is never removed, so the chromatids stay stuck and cannot separate to opposite poles. Note the chromosomes still condense (prophase happens before APC acts) and they are not fragmented — they simply fail to move apart. This exact reasoning was asked in NEET 2017.
The NCERT Cell Cycle and Cell Division chapter does not describe checkpoints or APC in its main text — these are NEET extensions built on the chapter's basics (S phase = DNA replication, anaphase = centromere splits and chromatids separate). But NEET has directly asked about them (2016 stalled fork, 2017 APC), so you must know the concept even though it is beyond the printed NCERT lines.
Anaphase promoting complex (APC) is a protein degradation machinery necessary for proper mitosis of animal cells. If APC is defective in a human cell, which of the following is expected to occur?
When cell has stalled DNA replication fork, which checkpoint should be predominantly activated?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Three main checkpoints are important for NEET: the G1/S checkpoint (before DNA copying), the G2/M checkpoint (before mitosis), and the M or spindle checkpoint (before anaphase, checking chromosome-spindle attachment).
APC acts at the metaphase-to-anaphase transition. When the spindle (M) checkpoint is passed, APC destroys the glue proteins between sister chromatids, triggering anaphase where the centromere splits and chromatids move apart.
No. Chromosome condensation happens in prophase, before APC acts. A defective APC only blocks the separation of sister chromatids (segregation) in anaphase; condensation still occurs normally.
A stalled fork means DNA replication is not proceeding correctly. The G1/S checkpoint guards entry into and progress through DNA synthesis, so it is activated to pause the cycle and prevent damaged or incomplete DNA from being carried forward.
APC and specific checkpoints are not described in the NCERT chapter's main text; they are NEET extensions. However, NEET has asked about them (2016 and 2017), so learn the concept even though it is beyond the printed NCERT lines.
