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Cell and Molecular > L6 - Damage, Repair and Recombination > Flashcards

L6 - Damage, Repair and Recombination Flashcards

1
Q

DNA can be damaged by

A

Oxidation
Hydrolysis
Methylation

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2
Q

What is hydrolytic deamination?

A

Most frequent and spontaneous
Cause point mutations in DNA sequences
Liberate an amine from its sugar attachment
- Deaminated C forms U so changes base pairing

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3
Q

What is hydrolytic depurination?

A

Most frequent and spontaneous
Cause point mutations in DNA sequences
Liberate a purine from its sugar attachment

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4
Q

What are pyrimidine dimers?

A

Caused by UV radiation
Cause formation of strong carbon-carbon bonds
Covalently attach pyrimidines
Can arrest DNA replication or cause misreading by DNA polymerase

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5
Q

What are DNA breaks?

A

Caused by ionising radiation
Double stranded breaks in DNA are dangerous
- Large fragments of DNA can be lost

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6
Q

What is base excision repair used for?

A

Deamination and depurination

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7
Q

Base excision repair mechanism

A
  1. Damaged base cleaved and removed by DNA glycosylase
  2. Sugar removed by apurinic/apyrimidinic endonuclease
  3. Phosphate removed by phosphodiesterase
  4. DNA polymerase extends primer:template junction
  5. DNA ligase seals nick
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8
Q

What is nucleotide excision repair used for?

A

Pyrimidine dimers

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9
Q

Nucleotide excision repair mechanism

A
  1. Excision nuclease cleaves single stranded DNA segment containing defect
  2. DNA helicase removes damaged segment
  3. DNA polymerase extends primer:template junction
  4. DNA ligase seals nick
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10
Q

What is non-homologous end joining used for?

A

Double stranded breaks

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11
Q

Non-homologous end joining mechanism

A
  1. Loss of nucleotides due to degradation from end
  2. Ends are then ligated together
  3. Loss of DNA sequence occurs
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12
Q

What is homologous recombination used for?

A

Double stranded breaks

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13
Q

Homologous recombination mechanism

A
  1. Broken DNA molecule resected at 5’ end
  2. Creates single strands used to prime DNA synthesis
  3. RecA promotes strand invasion of undamaged template molecule by one damaged primer strand
  4. Formation of a heteroduplex facilitates templated DNA synthesis of one strand by DNA polymerase
  5. Newly synthesised DNA dissociates from template
  6. Re-anneals to original partner strand allowing second strand synthesis
  7. Formation of a pair of staggered single stranded nicks - repaired by DNA ligase
  8. Information taken form undamaged DNA molecule and used to repair damaged one
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14
Q

Why is homologous recombination used?

A

Accurate
Last line of defence
- Single stranded breaks produced by excision repair may weaken DNA structure
- Then prone to becoming a double stranded break

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15
Q

What does defective nucleotide excision repair machinery cause?

A

Xeroderma pigmentosum
Patients sensitive to sunlight induced skin cancer
7 different genes - XPA, XPC, XPD, XPF, XPG
All encode proteins that participate in nucleotide excision repair pathway for correction of UV induced DNA lesions

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16
Q

What does mutations of homologous recombination DNA repair mechanisms cause?

A

Cancer
BRCA2 - breast, ovarian, prostate
ATM - Ataxia telangiectasia - leukaemia, lymphoma
Fanconi Aneamia - leukaemia

17
Q

BRCAs mutated cancers

A

Once homologous recombination is defective, cells become dependent on other pathways for correct DNA repair.
If both HR and BER DNA repair pathways are non-functional cells are unable to repair and die - synthetic lethality

18
Q

Homologous recombination in meiotic cells mechanism

A
  1. Spo11 endonuclease - initial cleavage
  2. Mre11 exonuclease - resects 5’ end
  3. Staggered double stranded break
  4. Resection of each 5’ end
  5. RecA - strand invasion
  6. DNA polymerase - DNA synthesis on both strands
19
Q

Each junction within a double holliday junction can resolve in one of two ways - for meiotic homologous recombination

A

Only internal strands broken and rejoined
- Same DNA strands broken and rejoined
External and internal strands broken and rejoined
-Different DNA strands are broken and rejoined
- Chromosomes have crossover

20
Q

Nucleotide Excision DNA repair is targeted to transcribed DNA sequences by the physical coupling of?

A

RNA Polymerase to the DNA repair machinery

Cell and Molecular (24 decks)

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