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Molecular Biology > Lecture 18 Repair of Mutations > Flashcards

Lecture 18 Repair of Mutations Flashcards

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1
Q

Describe the 4 mechanisms of Nucleotide repair

A
  • Direct repair
  • Excision repair
  • Mismatch repair
  • non homologous end joining
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2
Q

Explain what direct repair of a mutation would entail

A

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.

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3
Q

Explain why direct repair of damaged nucleotides is not that common

A

Uncommon because direct repair requires highly specific enzyme that can detect the nucleotide alteration caused by a mutation

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4
Q

Give two examples of direct repair to mutations

A

the A.D.A enzyme in E.coli can remove alkl groups from the

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5
Q

Which type of mutagenic agents cause mutations that can be repaired by direct repair

A

Alkylating agents

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6
Q

What does the ADA enzyme do and which type of mutation can it repair

A

ADA enzyme can come in and remove alkly groups from position 4 of thymine ring and position 6 of guanine

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7
Q

Give an example of an enzyme capable of direct repair in bacteria

A

ADA (in E.Coli)

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8
Q

Give an example of an enzyme capable of direct repair in humans

A

MGMT enzyme

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9
Q

What does the MGMT enzyme do

A

It is able to remove alkyl groups from position 6 of guanine via a direct repair mechanism.

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10
Q

Direct repair can fix …

A

The effects of alkylating agents (enzymes can remove alkyl groups
Base dimers due to UV light

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11
Q

State the role of DNA photolyase

A

recognizes base dimers and detaches bases that have been fused together by UV radiation

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12
Q

Which mechanism of DNA repair does DNA photolysase exhibit

A

Direct repair

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13
Q

Define and explain excision repair

A

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.

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14
Q

Name the two types of excision repair

A

Base excision repair

Nucleotide excision repair

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15
Q

Define base excision repair

A

a single base is removed

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16
Q

Define nucleotide excision repair

A

a longr piece of DNA containing the altered base is removed.

17
Q

What is the function of mismatch repair

A

Mismatch repair corrects the errors of replication.

18
Q

In which organism is the ADA enzyme found

A

E.Coli

19
Q

In which organism is DNA photolysase found

A

DNA photolyase is a bacterial enzyme. It separates base dimers formed by uv radiation induced base dimerization.

20
Q

define MGMT

A

Enzyme involved in the direct repair of alkylation mutations.

21
Q

Describe the base excision repair process, paying paticular attention to the mode of action of DNA glycosylases (in E.coli)

A
  • 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)
22
Q

Explain how nucleotide excision repair results in correction of mutations in E.Coli.

A
  • 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.
23
Q

Describe the importance of DNA methylation in the mismatch repair system of E.Coli.

A
  • 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.
24
Q

Outline how double stranded breaks are repaired by non-homologous end joining.

A

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.

25
Q

Which type of repair mechanism is the UvrABC complex involved

A

Nucleotide excision repair in E.Coli

26
Q

What is the role of the Uvr ABC complex in Nucleotide excision repair

A
  • Uvr A and B enter first, recognize helix distortion.
  • A departs, C enters.
  • C and B together cleave on either side of the damage and remove single strand damage.
27
Q

Which DNA strand usually contains the mismatched nucleotide (in mismatch repair)

A

The daughter strand

28
Q

How is the parent strand distinguished from the daughter strand? In E.Coli

A

In E.coli the parent DNA is methylated (before the daughter strand). (methylation of the daughter strand takes longer than the parent strand)

29
Q

What happens in humans, w.r.t to mismatch repair mechanism

A

In humans, the repair enzymes are suspected to be associated with the replication machinery, and mismatch detection occurs alongside synthesis of the strands. Thus DNA is immedietley scanned upon its synthesis for mis matched base pairing.

30
Q

State a common cause of double stranded chromosome breakages

A

Radiation

31
Q

How are natural chromosome ends distinguished from genuine breaks in the chromsomes

A

Due to presence of telomere repeat sequences, that protect the ends of chromosmes because telomeres are not recognized by the repair machinery as double stranded breaks.

32
Q

Describe the mechanism of non homologous end joining in humans

A
  • Double stranded break is recognized by the Ku proteins.
  • Ku proteins bind to the two broken ends.
  • Ku proteins attract eachother whilst keeping the two strands in place for repair.
  • DNA ligase then re-joins the two adjacent ends, ligating the ends together.
33
Q

What are Ku proteins and what is their function

A

They are involved in non homologous end joining repair mechanism.

34
Q

What is Non homologous end joining another word for

A

Double stranded chromosome breakages.

35
Q

What does DNA glycosylase do

A

Enzyme that cleaves/cuts the Beta-N-glycosidic bond between a base and sugar component of a nucleotide as part of base excision and mismatch repair process.

36
Q

What is the AP endonuclease

A

An enzyme involved in base excision repair.

Molecular Biology (19 decks)

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