Molecular Biology Lecture 9 - DNA Repair Flashcards
DNA repair - E. coli
- Direct reversal of DNA damage (pyrimidine dimers)
- Mismatch repair
- Excision repair
- base excision repair
- nucleotide excision repair
- transcription-coupled nucleotide excision repair
- Recombinational repair (double-strand break repair)
- Translesion polymerase
Direct reversal of DNA damage
- UV light causes thymine dimers
2. Photolyase, activated by violet/blue light, reverses the reaction
Another Direct reversal of DNA damage
Methyltransferase removes methyl group from O6-methylguanine
Mismatch repair
- MutS recognizes mismatch distortion
- Recruits rest of the complex
- SS endonclease followed by exonuclease
- DNA pol III and DNA ligase
How does repair complex tell the mutant nucleotide from the correct nucleotide?
dam gene (DNA adenine methylase)
- methylates A in GATC
- Fig 9-5 is incorrect
- daughter strand is initially not methylated
Unmethylated daugher strand is repaired.
Must occur before daughter strand is methylated.
- MutH recognizes unmethylated GATC
Base-excision repair
- For situations in which base has been changed or base analog has been inserted.
- Specific glycosylase removes abnormal base (e.g. uracil, inosine, 8-oxoG.)
- specific glycosylase for each abnormal base
- Abasic (apurinic or apyrimidinic) ribose is removed by nuclease.
- DNA polymerase and DNA ligase repair gap.
Base-excision repair
- What if DNA replication puts an A across form 8-oxoG?
- Specific glycosylase removes the A.
- Abasic site repaired to C.
- 8-oxoG still present.
Base-excision repair
- What if DNA replication puts an A across form 8-oxoG?
- Specific glycosylase removes the A.
- Abasic site repaired to C.
- 8-oxoG still present.
Nucleotide-excision repair
- Recognizes large distortion in DNA (modified base, thymine dimer)
- Repair protein recognizes distortion.
- DNA is locally denatured.
- Endonuclease cuts DNA on both sides of the distortion
- Gap is repaired by DNA polymerase and DNA ligase
Nucleotide-excision repair
- Important in repairing UV damage in humans.
- Xeroderma pigmentosum
- extreme sun-induced freckling, skin damage, skin cancer
- Mutations in one of seven genes involved in either excision- repair or transcription-coupled excision repair pathway.
Transcription-coupled nucleotide-excision repair
- Transcription is stopped or slowed by large distortions (e.g. thymine dimers).
- One way of recognizing mutations.
- Excision repair proteins are recruited and lesion is repaired.
DNA repair
- Direct reversal of DNA damage
- Mismatch repair
- Excision repair
- base excision repair
- nucleotide excision repair
- transcription-coupled nucleotide excision repair
- Recombinational repair (double-strand break repair)
- Non-homologous end joining
- Translesion polymerase
Double-strand break (DSB) repair
- Many repair mechanisms rely on copying information from a wild-type sequence.
- mismatch and excision repair: copies template strand
- DSB repair
- both strands are damaged
- copies sister chromatid
- S and G2 in eukaryotes
- homologous chromosomes are not often used as repair templates
- DSB repair uncommon in G1
- DSB repair is integral part of homologous recombination
DNA repair
- Direct reversal of DNA damage
- Mismatch repair
- Excision repair
- base excision repair
- nucleotide excision repair
- transcription-coupled nucleotide excision repair
- Recombinational repair (double-strand break repair)
- Non-homologous end joining
- Translesion polymerase
Double-strand break (DSB) repair
- Many repair mechanisms rely on copying information from a wild-type sequence.
- mismatch and excision repair: copies template strand
- DSB repair
- both strands are damaged
- copies sister chromatid
- S and G2 in eukaryotes
- homologous chromosomes are not often used as repair templates
- DSB repair uncommon in G1
- DSB repair is integral part of homologous recombination
