Lecture 18 Repair of Mutations Flashcards
Describe the 4 mechanisms of Nucleotide repair
- Direct repair
- Excision repair
- Mismatch repair
- non homologous end joining
Explain what direct repair of a mutation would entail
Direct repair systems act directly on the damaged nucleotide, converting each one back to its original structure. Direct repair mechanism requires specific enzymes per nucleotide.
Explain why direct repair of damaged nucleotides is not that common
Uncommon because direct repair requires highly specific enzyme that can detect the nucleotide alteration caused by a mutation
Give two examples of direct repair to mutations
the A.D.A enzyme in E.coli can remove alkl groups from the
Which type of mutagenic agents cause mutations that can be repaired by direct repair
Alkylating agents
What does the ADA enzyme do and which type of mutation can it repair
ADA enzyme can come in and remove alkly groups from position 4 of thymine ring and position 6 of guanine
Give an example of an enzyme capable of direct repair in bacteria
ADA (in E.Coli)
Give an example of an enzyme capable of direct repair in humans
MGMT enzyme
What does the MGMT enzyme do
It is able to remove alkyl groups from position 6 of guanine via a direct repair mechanism.
Direct repair can fix …
The effects of alkylating agents (enzymes can remove alkyl groups
Base dimers due to UV light
State the role of DNA photolyase
recognizes base dimers and detaches bases that have been fused together by UV radiation
Which mechanism of DNA repair does DNA photolysase exhibit
Direct repair
Define and explain excision repair
The section containing the damaged nucleotide of the DNA is removed.
Excision repair is only possible because an intact template is preserved.
The other strand serves as the template which is intact
The missing section is re-synthesized to give the correct strand.
Name the two types of excision repair
Base excision repair
Nucleotide excision repair
Define base excision repair
a single base is removed
Define nucleotide excision repair
a longr piece of DNA containing the altered base is removed.
What is the function of mismatch repair
Mismatch repair corrects the errors of replication.
In which organism is the ADA enzyme found
E.Coli
In which organism is DNA photolysase found
DNA photolyase is a bacterial enzyme. It separates base dimers formed by uv radiation induced base dimerization.
define MGMT
Enzyme involved in the direct repair of alkylation mutations.
Describe the base excision repair process, paying paticular attention to the mode of action of DNA glycosylases (in E.coli)
- damaged base is removed using a DNA glycosylase.
- produces a gap in DNA - AP site is formed due to loss of base (same as heat/water damage)
- AP site recognized by an enzyme (endonuclease and phosphodiesterase)
- single nucleotide gap is filled in by DNA polymerase and DNA ligase (seals strand)
Explain how nucleotide excision repair results in correction of mutations in E.Coli.
- Section of the DNA with mutation is removed.
- Damaged nucleotides often cause helix distortion.
- Distortion is recognized by UVrA.
- UvB and UvrC excise segment.
removal of excised single strand by DNA helicase II. - UVr B bridges the gap.
- ## DNA polymerase I and DNA ligase act in the 5’3’ direction and synthetizes new strand.
Describe the importance of DNA methylation in the mismatch repair system of E.Coli.
- Parent strand is methylated first, thus allowing identification of daughter strand (containing mismatched base)
- MUT H and MUT S enzymes recognize the daughter strand as it is not methylated.
- mis match repair occurs before daughter strand methylation.
- Mut S attaches to the unmethylated daughter strand
- Mut H cuts the DNA.
- Mismatch is excised
- DNA is resynthesized by DNA polymerase I and DNA ligase.
Outline how double stranded breaks are repaired by non-homologous end joining.
Ku Proteins (x2) bind to the two broken ends.
Ku proteins attract eachother, whilst holding chromsomes in the correct place
DNA Ligase ligates the two adjacent ends.
