Question

If a cell suffers damage to its DNA while in S phase, how can this damage be repaired before the cell enters mitosis?

Short answer

Answer: A cell detects and repairs DNA damage during the S phase by activating cell cycle checkpoints, specifically the G2/M transition checkpoint, which prevents the cell from entering mitosis until the damage is repaired. Several DNA repair pathways, such as base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR), can be activated to repair the damage. If the damage is too severe and cannot be repaired, the cell may undergo programmed cell death or apoptosis, preventing the passage of damaged DNA to daughter cells.

Step-by-step solution

Understanding the Cell Cycle

The cell cycle has four phases: G1, S (synthesis), G2, and M (mitosis). During the S phase, DNA replication occurs, and each chromosome is replicated to produce two identical sister chromatids. If a cell's DNA becomes damaged during the S phase, several mechanisms can prevent this damaged DNA from being passed to the daughter cells during mitosis.

Identifying DNA Damage

Firstly, the cell must detect the damaged DNA. Specialized protein complexes called checkpoint proteins constantly monitor the cell during the cell cycle to identify any DNA damage, incomplete replication, or other irregularities. If they detect any issues, they will activate the cellular response required to repair the damage or halt the cell cycle.

Cell Cycle Checkpoint Activation

When checkpoint proteins detect DNA damage during the S phase, they activate the G2/M transition checkpoint, preventing the cell from entering mitosis until the damage has been repaired. The G2/M checkpoint ensures that the cell does not pass damaged DNA to the daughter cells, thus maintaining genomic integrity.

DNA Repair Mechanisms

Several DNA repair pathways can be activated to repair the damage that occurs during the S phase, such as base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR). These pathways work by recognizing specific types of DNA damage and repairing them using specialized enzymes and cellular machinery.

Apoptosis If Repair Is Not Possible

In some cases, the DNA damage may be too severe to be efficiently repaired. If the repair pathways cannot repair the DNA before the cell reaches the G2/M checkpoint, the cell may undergo apoptosis, which is a controlled, programmed cell death. This helps to ensure that damaged cells are removed from the organism, preventing the potential for the propagation of genetic mutations. In conclusion, if a cell suffers DNA damage during the S phase, it can be repaired before entering mitosis through the activation of cell cycle checkpoints and DNA repair pathways. If the damage cannot be repaired, the cell may undergo apoptosis to prevent the passage of damaged DNA to daughter cells.