DNA Repair Flashcards
How can damaged be induced?
Spontaneously = depurination and deamination
Mutagen-induced
What are the different mutagen0induced DNA damaged?
Pyrimidine dimers
Alklylation
Substitution, deletion, insertion
Frameshift mutations
Double-strand breakd
What are the two types of point mutation?
Transition
Transversion
What is a transition point mutation?
Purine or pyrimidine is replaced by another of the same kind
What is a transversion point mutation?
Purine or pyrimidine is replaced by the other kind
What do hydrolytic attacks cause?
Depurination
Deamination
What is depurination?
Loss of purine base = A&G from DNA leaving a abasic site
What happens if depurination and deamination is left uncorrected?
Lead to deletion or substitution of base pairs during DNA replication
What happens in deamination?
Deamination of bases in DNA yields unnatural nucleotides
These unnatural nucleotides, which can be directly recognized and removed by specific DNA glycosylases
What does deamination of C produces what?
C»_space;> U which can be repaired by uracil DNA glycosylase
What is the role of nitrous acid?
Oxidatively deaminates primary amines = producing transition mutations
How is C converted to T?
Cytosine is methylated to 5-methyl cytosine
5-methyl cytosine deaminated to T
What can happen when cells are exposed to UV radiation?
Formation of a dimer = between 2 pyrimidine bases
Occurs between two adjacent T or C
Name alkylating agents
Nitrogen mustard
EMS
MNNG
What are alkylating agents?
Chemical that add an alkyl group to another molecule
What problems can nitrogen mustard cause?
N mustard can cross-link w DNA at N7 position of G
Results in chromosome break
What happens to guanine bases when exposed to EMS and MNNG?
EMS yiels O6-ethylguanine
MNNG yields O6-methylguanine
Both of which can pair with T (instead of C)
What mutations do intercalating agents cause?
Insertion and deletion mutations = frameshift mutations
How do intercalating agents work?
They increase the distance between 2 consecutive base pairs
Replication of such DNA generates deletion or insertion of one or more nucleotides int eh newly synthsized DNA
Give an example of intercalating agent
Ethidium bromide binds to DNA
Missense vs Nonsense mutation
A missense mutation is a type of point mutation in which a single nucleotide change in the DNA sequence leads to the substitution of one amino acid for another in the protein sequence.
A nonsense mutation is a type of point mutation in which a single nucleotide change in the DNA sequence introduces a premature stop codon (nonsense codon) into the mRNA transcript.
What does the Ames test assess?
Mutagenicity of compounds
How does the Ames test work?
The bacteria is a auxotroph = meaning it cannot make Histidine so needs it to grow
Mix the test compound (potential mutatgent) and bacteria together
If there are colonies growing, the test compound is a mutagen
Need a control to compare basal mutation level
Name repair pathways for DNA synthesis error
Proofreading by DNA polymerase (exonuclease)
Mismatch repair (mutSLH)
Name repair of DNA modifications
Direct reversal of damage (photoreactivation repair)
Excision repair = base or nucleotie
By DNA glycosylase or apurinic/apyrimidinic (AP)
Name repair of replication fork barriers
Translesion synthesis
Name repair of breakd in DNA
Repair double-strand breaks by
Homologous recombination
Non-homlogous end joining
What happens in proofreading?
3’ to 5’ exonuclease
Checks from the newest base added
If incorrect, removes it
What is the job of midmatch repair?
Correct errors that remain after proofreading
How does mismatch repair work in E.coli?
MutH endonuclease makes a nick on 5’ side of unmethylated GATC (newly synthesized)
UvrD (helicase) and exonuclease removes defective strand
Unmethylated strand is corrected by DNAp III
What does Dam methylase do?
Enzyme found in bacteria that adds a methyl group to the adenine base in the DNA sequence GATC
What does photoreactivation repair do?
Removes pyrimidine dimers = C + T
How does photoreactivation repair work?
Occurs only in bacteria
PHOTOLYASE = photoreactivation enzyme
Cleaves bonds between pyrimidine = light-dependent reaction
What enzymes are needed for base excision repair (BER)?
DNA glycosylase cleaves the glycosidic bond = leaving apurinic or apyrimidinic site
AP endonuclease cleave the phosphodiester backbone at AP site
DNAp I and DNA ligase = restore base
What does nucleotide excision repair require to function?
ATP and UvrA-D
How does nucleotide excision repair occur?
UvrAB recognize damage
UrvBC nicks DNA
Urv D unwinds the marked region
Why might nucleotide excision repair be needed and what diseases are caused without it?
UV-induced DNA lesions
Xeroderma pigmentosum and Cockayne syndrome = unable to repair UV-induced DNA lesions
When is translesion DNA synthesis or ‘Error-prone repair’ used?
When a lesion is encountered during replication = DNAp III is replaced by error-prone translaesion DNAp
Pol IV or V
What does the translesion DNAp do?
Extend DNA synthesis beyond the T dimer independent of base pairing
Has no proofreading exonuclease activity
What is non-homologous end joining and how does it work?
Accidental ds-break = loss of nucleotides due to degradation from end joining
End recognition by KU heterodimers
Deletion of DNA sequence = emergency repair
