DNA repair II

Question 1

unnatural ds DNA breaks

Answer 1

produced by ionising radiation, oxidative DNA damage, replication fork arrest

Question 2

natural ds DNA breaks

Answer 2

involved in meiosis, yeast mating type, V(D)J recombination; Ig and T cell receptor genes and in type II topoisomerase action

Question 3

non-homologous end joining

Answer 3

broken ends ligated together, involves base loss

Question 4

homology directed repair

Answer 4

homologous chromosome/sister chromatid used as template for repair; exact repair

Question 5

non-homologous end joining components

Answer 5

• main form of DSB in vertebrates
• both Ku70 and Ku86, plus XRCC4/DNA ligase complex essential for repair
• Ku (or associated DNA PKcs) defects= immunodeficiency and lymphoma
• Ku dumer has ring structure, binds, bridges and protects broken DNA ends

Question 6

non-homologous end joining process

Answer 6

• ds break
• resection of ss by exonuclease
• DNA strands brought together; possible limited base pairing between them
• strands filled in; joined by ligation
• double helix reconstruction
• several bps present in original WT sequence msising

Question 7

homologous repair process

Answer 7

• DNA end resection by exonuclease
• base pairing with unwound DNA of sister chromatid
• strand extension with DNA of undamaged sister chromatid
• disengage and pair, Holliday junction formation, branch migration and resolution
• restore WT helix

Question 7

homologous recombination

Answer 7

• universal biological process
• involves exchange of DNA sequences between two homologous chromosomes
• maintains genomic intergrity; DNA repair
• guides meiotic chromosome segragation

Question 8

HR in eukaryotic cells

Answer 8

• conserved-bacteria, yeast to man
• major DNA repair pathway in yeast
• eukaryotic RecA homologous ; Rad51 and Dmc1
• RAD51 knockouts are lethal
• HR repair occurs in S and G2; need identical template for repair
• resolving enzymes just characterised

Question 9

chromosome end telomeres

Answer 9

• for linear chromosomes, ends could be viewed as ds DNA break
• but don’t activate repair pathways

Question 9

what are telomeres?

Answer 9

special nucleoprotein structures, that cap and protect chromosome ends, involve nucleotide sequence TTAGGG repeated in tandem many times.

Question 10

what do telomeres do?

Answer 10

prevent chromosome degradation and end-to-end fusion.

Question 11

end of telomere problem

Answer 11

length of telomere is shortened with each cell division until 50-60 divisions and the cell becomes senescent.
therefore, they act as a molecular clock counting cell divisions.

Question 12

telomeres in germ, stem and immortal cancer cells

Answer 12

enzyme telomerase maintains telomere length, therefore, continues division