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Flashcards in DNA Repair Deck (18):
1

Methods of DNA proofchecking

H bonding, then DNA pol can "double check"

1. conformational changes in DNA pol
2. 3'-5' exonuclease activity (DNA pol)
3. strand directed mismatch repair (after rep)

2

Exonuclease activity

DNA pol can hydrolyze in the 3-5 direction that has just been covalently bound. mismatched nucleotides do not H bond and free OH groups in wrong position, leaving abnormal geometry, inducing cleavate.

DNA pol makes 1 in 100K , but in proofreading 1 in 10bil

3

DNA Damage

spontaneously or chemical, metabolics can produce damaging products

-mutations
-can become permenant in replication chain

4

Mechanisms of Repair

removal of bases and replaced with new DNA
a base excision
b mucleotide excision
c mismatched repair

or double stranded breaks
a NHEJ
b HR

5

Base Excision Repair

DNA glycosylases

deamination events to remove unnatural bases or repair missing bases (depurinations)

travel along DNA helix to probe

lesion specific

8 different types

1. recognition
2. glycosylase cleaves off base from deoxyribise
3. abasic sugar left behind
4. AP endonuclease removes the sugar phosphate backbone
5. DNA pol adds new nucleotides, ligase seals

6

Nucleotide Excision repair

NEr recognizes distortions (damaged bases, pyrimidine dimers)

1. recognition, by multienzyme complex for distortion
2. nuclease cleaves on both sides including lots of bases
3. helicase separates the two strands
4. gap repaired DNApol, ligase

7

Strand directed Mismatch repair

used for mistakes made by DNA pol proofreading
-looks for distortions caused by misfit of non complimentary base pairs

1. MSH binds to bulge
2. Mlh binds to Msh at the DNA and scans for a nick (where Okazaki frag will be)
3. Mlh trigger dgredation all the way back to mismatch
4. gap filled by pol and ligase

8

HR

double stranded break
-primarily used to repair DS breaks during cell division when duplicated chromosomes have not split

1. single strand end invade the other double helix of the sister chrom or hom chromosome
2. ss DNA strands from damaged use complimentary of undamaged as a template to elongate 5-3.
3. elongation complete, disengage and pair with DNA pol and ligase

G2 cell cycle only when sister chromatids available

9

NHEJ

exonuclease activity removes protion of ends
3' ends pair with eachother, using others as template.
ligase fills gap.

missing bases are introduced.

ERROR PRONE
3' ends must overlap. loss of one or more bases during resection

10

DNA DAMAGE

endo
deamination
base hydrolysis
oxidation

Exo
UV damage
Carcinogens
-DNA adducts

11

deamination

nitrous acid or nitrite (preservative in food) alters structure for incorrect pairing

cytosine to uracil 100bp/genome/day

uracil DNA glycosylase to remove bases

12

Base hydrolysis

glycosidic bond between purine base and sugar in a nucleotide can spontaneously hydrolyze leavine the ribose sugar without a base (apurinic site

dypyrimidination does also occur but lower rate.

loss of base with no repair: DELETION

13

Oxidation

metabolites release Oxygen.

causes mispairing by adding O to bases, ruins h bonding

14

UV radiation

pyrimidine dimers. adjacent thymine. CYCLOBUTANE ring.

repair: NER, covalent bond requires NER for fix

15

carcinogens

Add bulky adducts to DNA.

NER repair, but if too bulky it is ineffective.

16

Hereditary Non polyposis colon cancer

HNPCC
thousands of polyps in colon

defective gene in strand directed mismatch repair. Msh1 in most cases.

progression from an adenoma to carcinoma can take place in 2-3 years rather than 8-10;

17

xeroderma pigmantosum

herreditray
inability to repair UV radiated DNA. 1000x more likely to develop skin cancer and REALLy more for tip of tongue

18

familial breast cancer

BRACA1 BRACA2
hereditary, mutations in dsDNA break repair.

with mutation = 40-80% more likely