DNA repair Flashcards
spontaneous base loss repaired via ______
Base excision repair
Spontaneous deamination repaired via __________
Base excision repair
Spontaneous deamination
- amine group replaced with oxygen
- can result in point mutation (C–>T)
Sunlight (UV)-induced thymidine dimers repaired via ________, ________, and _________
Direct reversal
Nucleotide Excision Repair (NER)
Trans Lesion Synthesis (TLS)
Base alkylation repaired via ________, _________, and _______
Base Excision Repair
Direct Reversal
Mismatch Repair
Base oxidation by ROS repaired via _________ and ___________
Base Excision Repair
Mismatch Repair
Insertion/deletion of nucleotides on DNA repaired by ________
NER
Bulky chemical adducts on DNA repaired by ______ and ______
NER
TLS
Mismatch replication errors on DNA corrected by _________
MMR
Intra/Inter-strand crosslinks on DNA corrected by ______ and _______
NER
Single Strand Break Repair
DNA strand breaks repaired by _______, _________, and ________
Direct reversal
SSBR
DSBR
Direct Reversal Repair used to fix what kind of damage? (3)
Base Alkylation
UV-induced Thymine dimers
DNA strand breaks
Photolyase
reverses UV-caused base damage (T-T and T-C dimers)
Methyltransferase (MGMT)
-mutation results in?
- reverses base alkylation of O6-methylguanine
- mutation in MGMT reduces DNA repair activity and is associated with tumors
Mismatch repair (MMR) repairs these 3 types of DNA damage
Base alkylation
Base oxidation
Mismatch replication errors
Mismatch remair
- removes mis-incorporated nucleotides
- distinguishes between new strand and old strand
- copies info from intact DNA strand to the damaged strand
MMR distinguishes between template (old) and new strand by…
ecoli: methlyation of old strand
euk: knicks on new daughter DNA
Steps of MMR (3)
1) two protein complexes (hMSH and hMLH/PMS) recognize damage on new daughter strand
2) stretch of DNA in front of and behind damaged base clipped by endonucleases and excised by exonucleases
3) DNA pol synthesizes new DNA, sealed by ligase
Mutation in MMR can lead to ___________ (disease)
Herditary non-polposis colorectal cancer (HNPCC) aka Lynch Syndrome
5 general steps of excision repair (MMR, BER, NER)
1) recognition of damaged/mismatched nucleotide
2) Endonucleases cut phosphodiester backbone flanking damaged nucleotide
3) Nucleases remove DNA fragment containing damaged nucleotide
4) DNA polymerase synthesizes missing nucleotide by using intact DNA strand
5) DNA ligase seals remaining nick in phosphodiester backbone
Base Excision Repair (BER) repairs these types of DNA damage (4)
repairs damages that do NOT distort the DNA
- Spontaneous Base Loss
- Spontaneous Deamination
- Base Alkylation
- Base Oxidation
Base Excision Repair
- Uses base-specific glycolases to remove damaged base
- limited based on # of specific glycolases
- repairs damages that do NOT distort the DNA
Steps (4) of base excision repair
- Damaged base recognized by specific DNA glycolase
- Damaged base flipped out and glycolase cuts N-glycosidic revealing AP site
- AP site-specific endonuclease (APE1) cleaves sugar-phosphate backbone at 5’ and another cuts at 3’
- Gap filled by DNA polymerase and knock sealed by DNA ligase
Nucleotide Excision Repair repairs these types of DNA damage
- UV damage thymine dimers
- Nucleotide insertion/Deletion
- Bulky chemical adducts
- Inter/Intra strand crosslinks
NER
- repairs base damages that distort the DNA and blocks polymerase function
- removes oligonucleotide that contains the damaged base
NER steps (5)
1) recognition of site by multiprotein complex (global genome NER, TRanscription-coupled NER)
2) local unwinding by DNA helicase to form a bubble
3) Double incision of damaged strand by two endonucleases
4) Removal of 30 base oligonucleotide containing the lesion
5) fill in gap by DNA pol, rejoin by DNA ligase
Global genome NER
-mutations result in…
- recognizes damage anywhere in the genome
- Mutations can result in cancer, and XP
Transcription-Coupled NER
-mutations result in…
- recognizes damage within transcribed region
- Mutations can result in CNS disorder, Cockayne syndrome (CS)
Homologous recombination (HR)
- Extended sequence homology needed
- accurate
Non-homologous end joining (NHEJ)
- No homology needed
- often innacurate
Single Strand Break Repair (SSBR)
- PARP activated by ssDNA break
- adds poly-ADP-ribose chain to proteins that facilitate DNA repair (amplified damage signal)
Tolerance/Bypass of base damage (trans-lesion synthesis)
- Last resort when too much DNA damage
- Uses error prone DNA polymerases (without proofreading mechanisms)
- Allows replication to continue on damaged template strand
Mutations incorporated into new strand
Steps (5) of trans-lesion synthesis
1) DNA replication machinery stalls
2) Two “bypass” polymerases bind to arrested replication complex
3) Conformational change of replication complex allows Pol n (bypass polymerase) to synthesize new strand over the lesion
4) Pol I (bypass polymerase) extends new strand a little further beyond lesion
5) DNA replication resumes with normal machinery
DNA damage checkpoint
pauses cell cycle to allow time for DNA repair before moving on to the next step of the cell cycle
