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Medical Cell Biology and Genetics > L15- DNA repair > Flashcards

L15- DNA repair Flashcards

1
Q

DNA integrity can be looked at in 2 ways

A
  • at the level of nucleotdie- mutations
  • at the level of chromsosomes (nondisjunction)
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2
Q

if DNA integrity is lost

A

pathogenic

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3
Q

types of DNA breaks

A

Single stranded

double stranded

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4
Q

single stranded damage

A

One strand will remain intact

Not as bad as DS break

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5
Q

double stranded damage

A

breaking the double strand is very damage

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6
Q

which compoenents of DNA can be dmaaged

A

all components e.g. bases and sugar phosphate backbone

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7
Q

what causes DNA damage

A

exogenous and endogenous sources

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8
Q

endogenous causes of DNA damage

A

error in DNA replication (replication stress)

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9
Q

exogenous sources of DNA damage

A

Anti-cancer Drugs

Mutagenic chemical - smoking

Alkylation agents

Radiation

Ionising – most comes from radon gas in the ground

Free radicals

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10
Q

what usually happens when healthy DNA becoems damaged

A

is damaged DNA can be recognised by DNA repair mechanisms which makes the DNA healthy again.

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11
Q

when does m utation occur

A

when DNA damage is not recognised or repair mechanisms dont work

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12
Q

endogenous sources of single stranded damage

A

i. Replication stress
ii. Free radicals

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13
Q

exogenous sources of single stranded damage

A

i. Deamination due to nitrites
ii. UV radiation

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14
Q

exogenous sources of double stranded dmaage

A

X-ray/Y-ray

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15
Q

definititon of replciation stress

A

‘Inefficient replication that leads to replication fork slowing, stalling and/ breakage.’

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16
Q

replication stress is an example of

A

an endogenous source of damage, which causes replication stress

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17
Q

how many factors causes replication stress

A

3

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18
Q

outline 3 factors which cause replication stress

A

1) Replication machinery defects

  • Misincorporation by DNA polymerase
  • Proofreading error by DNA polymerase

2) Replication fork hindrance

  • Forward and backward slippage
  • e.g. Trinucleotide repeats

3) Defects in response pathway

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19
Q

replication machinery defects

A

Misincorporation by DNA polymerase

Proofreading error by DNA polymerase

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20
Q

what leads to replciation fork hindrance

A

repetitive DNA

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21
Q

types of replication fork hindrance

A

forward slippage

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22
Q

backward slippage

A
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23
Q

what sort of mutation is forward slippage

A

deletion mutation

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24
Q
A
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25
Q

what type of mutation is backward slippage

A

insertion mutation

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26
Q

forward slippage mechanism

A

New strand has an extra nucleotide (A)

Newly synthesised strand loops out

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27
Q

backward slippage mechanism

A

New strand is missing a nucleotide (A)

Template strand loops out

28
Q

backward fork slippage leads to

A

trinuclotide expansion seen in Huntingotns and fragile x syndrome

29
Q

Huntingtons disease is a

A

trinucleotide repeat disorder

30
Q

which gene is affected in huntingtons

A

HTT gene

31
Q

which codon affected

A

CAG repeat- polyglutamine

32
Q

CAG repeats cause

A

neurone degenration

  • mutant protiein aggregates in neurones affecting basal ganglia
33
Q

when DNA damage levels are too high or persist the cell can …. (3)

A
  • goes into senescence (G0)
  • undergoes apoptosis
  • undergoes uncrontrolled cell division- tumour
34
Q

when do the cell cycle checkpoint occur

A

G1 When entering the S phase

G2 checkpoint

Checkpoint in. mitosis

35
Q
A
36
Q

G1 checkpoint

A

is the environment favoruable to enter DNA replication

37
Q

G2 checkpoint

A

is all DNA replicated?

is all DNA damage repaired?

38
Q

checkpoint in mitosis

A

are all chromosmes properly attache to the mitotic spindle?

39
Q

repair mechanisms for single stranded breaks in DNA

A

1) Mismatch repair
2) Base Excision repair
3) Nucleotide excision repair

40
Q

what is G0 (senecence)

A

Permanent cell cycle arrest- will never divide again (wont pass bad DNA), but will continue with normal function

41
Q

mismatches occur due to

A

endogenous errors during replication

  • DNA polyemerase adds wrong nucleotide (happens 1/100,000 nucleotides)
42
Q

what proofreads DNA and reduces error rate

A

Exonucleases- attached to DNA polymerases -checks the for mismatches and replaces with correct nucleotide

Reduces error rate

43
Q

DNA repair mechanism for endogenous causes of ssDNA breaks

A

mismatch repair mechanism

44
Q

mismatch repair emchanism

A
  1. Mismatch is detected in newly synthesised DNA
  2. New DNA strand is cut and mispaired nucleotides and neighbour removed
  3. Missing patch is replaced with correct nucleotides by DNA polymerase
  4. A DNA ligase seals the gap in DNA backbone
45
Q

exogneous causes of ssDNA breaks

A

harmful chemical or physical agents

46
Q
A
47
Q

harmful chemicals

A

Nitrites and nitrosamines in cured or pickled food

48
Q

Nitrites and nitrosamines in cured or pickled food cause

A

deamination

e.g. Removal of amino group from nitrogen bases (change cytosine à uracil)

49
Q

DNA repair mechanism when deamination occurs

e.g. Removal of amino group from nitrogen bases (change cytosine à uracil)

A

base-excision repair

50
Q

base excision repair

A

1) Converts a cytosine base into a uracil
2) The uracil is detected and removed, leaving a base-less nucleotide
3) The base-less nucleotide is removed, leaving a small hole in the DNA backbone
4) The hole is filled with the right base by a DNA polymerase and the gap is sealed by a ligase

51
Q

physical agent swhich cause ssDNA damage

A

UV radiation

52
Q

UV radiation causes

A
  • UV radiation creates pyrimidine dimers (most often with 2 adjacent thymine)
  • Thymines forms bonds with eachother distorting the whole DNA strand
53
Q

what is used to repair ssDNA dmaage caused by UV radiation

A

nucleotide exicision repair

54
Q

nucleotide exicision repair

A

1) UV radiation produces thymine dimer
2) Once dimer has been detected, the surrounding DNA is opened to form a bubble
3) Enzymes (exonucleases) cut the damaged region out of the bubble
4) A DNA polymerase replaces the excised (cut-out) DNA and ligase seals the backbone

55
Q

dsDNA breaks cause

A

most delterious damage

56
Q

what causes ds breaks

A

exognous cause: ionising radiation (X-ray and Y-ray)

57
Q

X-ray and Y-rays

A
  • Can break phosphate backbone of both DNA strands
  • Causes double stranded breaks
  • Usually not a clean break- often an over hanging region
58
Q

DNA repair mechanissm for ds breaks

A

1) Non-homologous end joining
2) Homologous directed repair

59
Q

ssDNA repair meechanism

A
  • mismatch
  • base excision repair
  • nucleotide exicision repair
60
Q

non homologus end joining

A
  1. DNA protein kinase(KU70/80) binds to each end of the broken DNA and recruits artemis
  2. Artemis cuts off jagged ends
  3. Ligase enzymes binds two ends of DNA
61
Q

non homologus end joining involves

A

loss of some geentic information

  • error prone repair
62
Q

how does homolgous recombination occur

A

We have 23 pairs of homologous chromosomes that have similar genetic material and code for the same traits- similar nucleotide sequences… therefore a double stranded break can be repaired using a sister chromatid

63
Q
  1. outline homologous recombination
A

A protein complex called MRN binds to each end of the broken DNA and recruits exonucleases which remove nucleotides from one strand of the DNA

End 1 is placed next to a similar nucleotide sequence called the homologous sequence and starts to base pair with complementary region

This creates a loop in the homologous DNA

DNA polymerase synthesises nucleotides to extend end 1 until it reaches a sequence which is complementary to end 2

End 1 releases the homologous DNA and its last few nucleotides bind to the last few nucleotides of end 1

DNA polymerase fills the gap on both sides of the union and DNA ligase seals the bond

64
Q

homologus recombination is

A

more reliable than non-homolopgus end joining

  • no loss of genetic info
65
Q
A

Medical Cell Biology and Genetics (33 decks)

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