Question

Q1: If DNA damage occurred, the cells repair the damage by several mechanisms during the cell cycle.

In which phase of the cell cycle cells will repair the damage?

Name five of the repair mechanisms that cells are using to repair DNA damage?

What are the repair mechanism for double strand DNA breaks? Describe them and Which of them is the error- free repair pathway?

Answer 1

1. DNA damage checkpoint is a signal transduction pathway that blocks cell cycle progression in G1, G2 and metaphase and slows down the rate of S phase progression when DNA is damaged. It leads to a pause in cell cycle allowing the cell time to repair the damage before continuing to divide.

2.  Five major DNA repair pathways—base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), homologous recombination (HR) and non-homologous end joining (NHEJ)—are active throughout different stages of the cell cycle, allowing the cells to repair the DNA damage.

3. DNA double-strand breaks are repaired by means of two main mechanisms: nonhomologous end joining and homologous recombination.

Non-homologous end joining (NHEJ) is a pathway that repairs double-strand breaks in DNA. NHEJ is referred to as "non-homologous" because the break ends are directly ligated without the need for a homologous template.

Homologous recombination repair is a DNA repair process that includes the invasion of an undamaged DNA molecule by a damaged molecule of identical or very similar sequence. Resynthesis of the damaged region is accomplished using the undamaged molecule as a template.

Non-homologous end-joining NHEJ is an error-prone alternative to HR repair that can also be employed to repair double strand breaks. Since NHEJ does not use a homologous DNA template, such as a sister chromatid or a homologous chromosome, it often results in the insertion or deletion of new nucleotides at the fused DSB junction.