Chromosome biology lecture 8

Question 1

Genomic DNA is continuously degraded

Types of DNA damage

Answer 1

• DNA damage = through endogenous (hydrolytic/oxidative) or environmental (physical & chemical)
• Different types of damage = ss or ds breaks in sugar phosphate backbone, dirty/clean, DNA mismatches, missing bases, altered bases

Question 2

DNA mutations

Answer 2

• Deletions, insertions or substitutions
• Mutagenesis of single bases = transitions or transversions
• Large scale mutations have ↑ effects, loss of genetic info → loss of heterozygosity

Question 3

Frequencies of DNA damage

Answer 3

• Depurination = 12,000 per day, ds = 9
• Chemical agents used in chemotherapy
• Mutation = 1.1-2.5x10^-8 (compared to 50,000ss = very little, good repair)

Question 4

Strategies for DNA repair

Answer 4

1. Direct reversal of repair or damage

2. DNA damage tolerance + potential for mutagenesis

Question 5

Types of DNA damage

Answer 5

1. Spontaneous de-amination of C→U (2 ways could occur, U bp w/ A not G)
2. De-amination of other bases (e.g. nitrous acid reacts w/ amino group → deamination, A→hypoxanthine
3. H atoms on bases change (tautomer, amino tautomerises to imino, keto to enol)
4. Alkylating agents (mono functional or bifunctional, often nucleophilic centres)
5. Loss of bases through deprivation/depyrimidination (cleavage of N-B-glycosyl bond → abasic site

Question 6

Direct reversal of DNA damage

Answer 6

• UV radiation → CPD + 6-4PP

- Both can be reversed w/ DNA photolyases (contain non-covalently bound chromophores)

Question 7

Direct reversal of O6-G + O4-T alkylations

Answer 7

• O6-MGTI transfers methyl from O6 of G (DNA) to C (protein)
• 2 Cys acceptor receive methyl
• Once methylated, O6-MGT1 x be regenerated, finite damage that can be repaired
• Cont exposure to alkylating agents → mutations
• Methylated O6-MGTI = transcriptional activator, activates genes w/ Ada box (transcribes genes needed for BER)
• Humans = o6-AGT, also uses Cys