Compare and contrast nonhomologous end joining (NHEJ) and homologous recombination as methods of repairing double-stranded breaks in DNA

Question

Under what conditions or in what organisms is homologous recombination not a likely mechanism for repair of double-stranded breaks?
What will be an ideal response?

Answer 1

Answer: NHEJ involves the recognition of broken DNA fragments by the Ku80-Ku70 protein complex, the trimming of the ends, and the joining of the fragments by DNA ligase IV. NHEJ results in the deletion of nucleotides in the DNA strand and may thus result in frameshift mutations or other alterations of the genetic code. In addition, it is possible that two unrelated DNA fragments could be joined together, as there is no mechanism to ensure that the DNA fragments arose from the same piece of DNA.
Homologous recombination uses a homologous chromosome or stretch of DNA to fill in DNA that has been lost. Both strands of the broken fragment invade a stretch of homologous DNA, and the intact strands are used as a template to replace the DNA that was lost due to the double-stranded DNA break. Compared to NHEJ, homologous recombination is less error-prone and more likely to repair the DNA damage to a state similar to the original sequence (although perhaps not identical).
Homologous recombination is not as likely in organisms that are haploid and do not contain homologous chromosomes, such as bacteria and archaea. However, homologous recombination can occur in haploid organisms if there are regions of similarity within the genome, with extra-chromosomal elements, or if part of the genome has been duplicated before cell division.