4.2: Repair of DNA mutations

Why is DNA repair important?

For preserving the composition and coding potential of the genome 

Damage to DNA repair system can cause what?

Disease

What is the estimated frequency of DNA damage?

~20000-50000 lesions per cell per day

What are the 4 types of repair of mutations?

Direct / excision / mismatch / nonhomologous end joining

What is the most common type of DNA repair mechanism?

Excision repair

What is an ADA enzyme?

An Escherichia coli enzyme that is involved in the direct repair of alkylation mutation 

What is an AP endonuclease?

An enzyme involved in base excision repair

What is direct repair?

A DNA repair system that acts directly on a damaged nucleotide

What is a DNA glycosylase?

An enzyme that cleaves the B-N-glycosidic bond between a base and the sugar component of a nucleotide as part of the base excision and mismatch repair processes 

What is a DNA photolyase?

A bacterial enzyme involved in repair of base dimers 

What is DNA repair?

The biochemical process that corrects mutation arising from replication errors and the effects of mutagenic agents

What is Excision repair?

A DNA repair process that correct various types of DNA damage by excising and resynthesizing a region of polynucleotide

What does MGMT stand for?

O6-methylguanine methyltransferase

What is MGMT?

An enzyme involves in the direct repair of alkylation mutations

What is mismatch repair?

A DNA repair process that corrects mismatched nucleotides by replacing the incorrect nucleotide in the daughter polynucleotide

What 2 types of mutation can be repaired by direct repair?

Base dimers from UV/ alkylating agents

What enzyme can remove alkyl groups in E.coli?

ADA enzymes

Which enzyme can remove alkyl groups in humans?

MGMT enzyme

How and where are alkyl groups removed in humans?

By MGMT enzyme / at position 6 of G

How and where are alkyl groups removed in E.coli?

By ADA enzyme / at position 4 of T and 6 of G

Which enzyme can directly repair base dimers formed by UV radiation in E.coli?

DNA photolyase

Which enzyme can directly repair base dimers formed by UV radiation in humans?

None

Why is UV radiation so dangerous for humans?

Because humans dont have to enzyme to direct repair UV mutations

What are the 2 types of excision repair?

Base / nucleotide excision repair

What is base excision repair?

A single altered base is removed 

What is nucleotide excision repair?

A longer piece of DNA containing the altered bases is removed

How does excision repair begin in E.coli?

Begins with removal of the damaged base by a DNA glycosylase enzyme?

Which enzymes carries out base excision in E.coil (removal of base)?

DNA glycosylase

What is an AP site?

Apurinic / apyrimidinic site

What does base excision in E.coli result in?

Result in an AP site, which is then filled in 

What is an abasic site?

AP site

Which enzyme is required to fill in the AP site, as a result of base excision?

DNA polymerase / DNA ligase

Which enzyme carries out nucleotide excision repair in E.coil?

UvrABC endonuclease

What molecule first attaches to the damaged nucleotide during nucleotide excision repair?

UvrAB trimer 

What happens after UvrAB trimer attaches to damaged nucleotide in excision repair?

UvrA departs, UvrC attaches 

How are parent and daughter stranded distinguished in E.coli?

Parent strand is methylated 

Which enzyme can recognise a mismatch in E.coli?

MutH and MutS enzymes

What is ther difference in function of MutS and MutH

MutS recognise damage, MutH recognises the different strand and cuts DNA

Which enzymes are responsible for the strand detachment and removal for mismatch repair in E.coli?

Exonuclease / DNA helicase II

How do enzymes differentiate parent and daughter strands in humans?

Enzyme is highly associated with the replication fork, recognise daughter strand as it is being synthesised

How would you distinguish real breaks from the natural ends of chromosomes?

Telomeres mark the natural ends

How are non-homologous end joining carried out in humans?

Ku proteins join at ends of double strand breaks / ku protein attract one another / DNA ligase joins ends together

What molecule detects a double-stranded break?

Ku proteins