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: The importance of DNA repair during the S phase of the cell cycle is to ensure that the DNA is error-free before the cell enters mitosis, preventing the transmission of mutations to the resulting daughter cells. The main repair mechanisms used by the cell during this phase include Base Excision Repair (BER), Nucleotide Excision Repair (NER), Mismatch Repair (MMR), Homologous Recombination (HR), and Non-Homologous End Joining (NHEJ).

Step-by-step solution

The cell cycle is responsible for cell growth, replication, and division, and it consists of four main phases: G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis) phase. The S phase is when the DNA within the cell is replicated in preparation for cell division. If there is damage to the DNA during this phase, it must be repaired before the cell can move on to mitosis. #Step 2: Importance of DNA repair before mitosis#

It is crucial that the DNA is error-free before the cell enters mitosis to ensure that the resulting daughter cells have the correct genetic information. If damaged DNA is not repaired, it can lead to mutations, which may cause various diseases or defects in the organism. #Step 3: DNA damage response system in S phase#

The cell has a DNA damage response system in place to detect and repair damaged DNA during the S phase. The system involves the activation of checkpoint proteins, which halt the cell cycle progression, and the recruitment of DNA repair proteins to the site of the damage. #Step 4: DNA repair mechanisms#

There are multiple DNA repair mechanisms that can be employed by the cell to fix damaged DNA during the S phase, including: 1. Base Excision Repair (BER): BER is used to correct small, non-helix distorting base lesions, like oxidized or deaminated bases. The damaged base is removed, and a new base is added by DNA polymerase and then sealed by DNA ligase. 2. Nucleotide Excision Repair (NER): NER is responsible for repairing bulky, helix-distorting lesions such as those caused by UV radiation or chemical exposure. The damaged region is excised, and new DNA is synthesized using the undamaged strand as a template. 3. Mismatch Repair (MMR): MMR corrects base mismatches and insertion or deletion loops that can occur during DNA replication. The mismatched nucleotide is removed, and the missing bases are filled in by DNA polymerase and sealed by DNA ligase. 4. Homologous Recombination (HR) and Non-Homologous End Joining (NHEJ): Both HR and NHEJ are responsible for repairing DNA double-strand breaks. HR uses the sister chromatid as a template for accurate repair, while NHEJ directly joins the broken ends together. #Conclusion# To summarize, if damage occurs to a cell's DNA during the S phase, the DNA damage response system identifies and repairs the damage before the cell moves on to mitosis to avoid passing on any mutations. There are several DNA repair mechanisms at work during this phase, including BER, NER, MMR, and double-strand break repair through HR or NHEJ.