1-7 DNA Repair Flashcards
What are the different types of DNA damage?
Deamination, Depurination, Alkylation, Pyrimidine Dimer, Mismatch, Inserted Ribonucleotide, Base oxidation, bulky adduct, Single strand break, double strand break,
Deamination
cytosine to uracil. after replication will cause an AT pair instead of a CG pair in daughter
depurination
removal of purine from dna molecule
alkylation.
repaired by?
of guanine, makes it bind to thymine. duaghter strands will become A-T not C-G.
repaired by MGMT (direct reversal)
pyrimidine dimer
cross-linking between adjacent thymines by UV. distorts helix and blocks replication.
mismatch
incorrectly paired bases
inserted ribonucleotide
nucleotide in the sequence is a ribonucleotide instead of deoxyribonucleotide
base oxidation
extra hydroxl group bonded to a base
oxidized G pairs with A and causes CG to AT transverse mutation.
bulky adduct
large organic molecule is bonded to a nucleotide
single strand break
one strand of DNA molecule broken
double strand break
both strands broken
BER
steps
removes?
strands?
Base excision repair - Important for removing damaged bases that could cause mustations or mispairings or breaks in DNA during replication (repairs deamination, depurination, base oxidation, and single strand breaks.
- Initated by DNA glycoslyases that recognize altered/incorrect bases.
- glycosylase will remove the base, created an abasic (apurinic/apyrimidinic) site.
- AP endonuclease cleaves phosphodiester bond at 5’
- dTTP synthesized.
- Sealed with DNA ligase
endonuclease cleavage at one strand
NER
removes? enzymes?
strands?
Nucleotide excision repair - repair pyrimidine dimers and bulky adducts. Involves enzymes XPA through XPG.
Involves endonuclease cleavage a both strands.
MGMT
in non placental mammals
-directly repairs alkylation damage by scanning molecule
Defective BER is linked to?
cancer, neurological disorders
Defective NER leads to?
diseases such as xeroderma pigmentosum - caused by one or more enzymes of excision pathway deficiency.
RER
What enzyme used?
used to excise incorrectly placed ribonucleotides in the DNA molecule using RNase H2.
Most common type of DNA damage?
transient contamination of DNA with rNTP’s.
RER deficiency leads to?
Mutations arising from short deletions.
=Aicardi-Goutieres Syndrome
Mismatch Repair
repairs 99% of errors occuring during replication.
- complex of mismatch repair proteins scans newly replicated DNA for mismatches
- removes section of strand containing incorrect base
- resynthesizes missing strand
How does the DNA mismatch repair complex know where to look?
newly synthesized DNA contains gaps that have not been sealed yet - guiding the repair apparatus as to where to look for mismatches
mutation of any of the 6 genes that code for mismatch repair leads to
predisposition to certain cancers including the most common form of heredity colon cancer (hereditary non-polyposis colon cancer) (HNPCC)
Direct reversal
reverses alkylation. involves methyl guanine methyl transferase (MGMT). MGMT covalently transfers the alkyl group to it’s active cysteine cyte.
- inactivates MGMT
- restores guanine to normal
double strand break repair (??)
fixes ds breaks via homologous recom and non-homo end joining (NHEJ). Occurs when nick in DNA occurs prior to replication, leading to a long continuous strand ds, and then a blunted strand. ds
describe non homologous end joining (?)
in NHEJ, the two broken ends of DNA are joined via DNA ligase
- more error prone pathway but necessary when no template available
- favored by higher eukaryotes (humans)
Describe homologous recombination (?)
HR more accurate - uses a homo sequence (identical) as template to repair double strand breaks. Often used with dsb are created during replication
what is the DNA Damage Response?
major regulators?
DDR - pathway that involves signals, sensors, transducers, and effectors to accomplish cell cycle transitions, dna rep, dna repair, apoptosis.
-major regulators are the PI3K related Kinases (PIKKs) including ATM and ATR.
ATM
functions in response to rare DSBs.
ATR
activated during every S phase to
- regulate rep origin firing
- repair damaged forks
- prevent premature mitosis onset
Xeroderma Pigmentosum
defective NER (specifically because of XPA through XPG)
Hereditary Non-Polyposis Colon Cancer (HNPCC)
Due to mutations in any of the mismatch repair(MMR) genes. Develops the mutator phenotype (mutations accumulate in cells more frequently).
Aicardi–Goutières Syndrome (AGS) -
Mutations in the RNase H2 subunits, hindering RER. Due to short deletions. Rare neuroinflammatory disorder
Ataxia Telangiectasia
Due to homozygously inheriting mutations in the ataxia telangiectasia mutated (ATM) gene. Predisposes the individual to cancers and neurodegenerative diseases.
exo vs endo nucleases
endo - cut like scissors within DNA
exo - chew from outside.
5’-‘3 exo start at 5’
3-5’ exo start at 3’ end
what is a mutation?
permanent, heritable alterations in the base sequences of DNA
in BER, the lesion is removed by
dna glycosylase
distamycin
DNA groove binder. interacts with A-T in minor groove. Terminal end of molecule attracts the drug
Actinomycin
DNA intercalators - blocks chain elongation
cisplatin
dna crosslinking agent. anti-cancer for testicular and ovarian. crosslinks cause helix unwinding which prevent additional transcription/replication. eventual cell death
in NER, endonuclease makes nicks on either side of the lesion, which is then removed to make a gap which is filled by ____ to make the final phosphodiester bond.
DNA polymerase
What happens if the cell cycle checkpoint is reached, and DNA damage is not repaired.
DNA damage response (DDR) pathway is initiated. ATM is recruited to activate p53, which leads the cell to senescence or apoptosis.
Xeroderma pigmentosum patients lack one or more enzymes required for:
excision repair
The proofreading activity of DNA polymerase involves:
A. 3’–>5’ exonuclease function
Ultraviolet light produces thymine dimers in DNA strands. Excision repair requires all of the enzymes listed EXCEPT:
A. primase
B. DNA polymerase
C. DNA ligase
D. nuclease
A. Primase
HNPCC? caused by?
hereditary non-polypsis colon cancer - caused by mutation in any of the 6 mismatch repair genes
ussually only one allele inherited, doesnt appear until second allele mutates
autosomal dominant?
NER vs BER endonuclease activity
NER - both strands
BER - one strand
xeroderma pigmentosum
family of diseases caused mutations/deficiencies in one or more of the excision pathway enzymes
XPC, RPA, TFIIH, XPG, ERCC1-XPF
autosomal recessive
sensitive to UV light. damage not reversed
ataxia telangiectasia
caused by defect in DNA repair gene ATM
autosomal recessive
