DNA Damage and Repair Flashcards
(27 cards)
what is a mutation?
any change from the normal dna sequence
what are types of mutations?
deletions
substitutions
insertions
transitions
transversions
what is a deletion mutation?
loss of nucleotide
what is an insertion mutation?
gain of nucelotides
what is a substitution mutation?
change of one base for another
what is a transition nucleotide?
A to G (purine to purine)
C to T (pyrimidine to pyrimidine)
what is a transversion mutation?
purine to pyrimidine
what are some examples of exogenous (external) damage?
ionising radiation (x rays)
UV (sunlight)
chemicals
base modification and cross-linking between bases
what are some examples of endogenous (internal) damage?
dna replication errors
hydrolysis - water drives the reaction
oxidation
describe hydrolysis: deamination
loss of an amino group
cytosine to uracil
adenine to hypoxanthine
5 methylcytosine to thymine
describe hydrolysis: depurination
loss of adenine/guanine bases
each nucleated human cell loses ~5,000 dna purine each day
loss of pyrimidine is only 5% loss of purines
what are the consequences of dna damage?
cell death
functional decline of tissues
organ issues/failure
cancer
developmental deficiencies
embryonic lethality
what are the 7 dna repair mechanisms?
proofreading activity of dna polymerase
direct repair
base excision repair (BER)
nucleotide excision repair (NER)
mismatch repair (MMR)
non-homologous end joining (NHEJ)
homologous recombination repair (HR)
describe direct repair mechanism
infrequently used, 3 genes implicated
O6-methylguanine - dna methyltransferase, removes methyl grps
describe BER mechanism
dna glycosylases identify and remove the damaged base
AP endonuclease and phosphodiesterase cut the sugar-phosphate backbone
Gap is filled by dna polymerase b
nick is sealed by dna ligase
describe NER mechanism
damages bases are detected
helicases and nucleases act to open up and cut either side of the mutated bases
gap is repaired by dna polymerase e or d
nick is sealed by dna ligase
describe MMR mechanism in simple terms
correct the mismatched base pairs or small insertion/deletion loops caused by errors in dna replication
what is the function and use of MSH proteins?
form ‘sliding clamps’ that move along the dna to identify damage
targeted by MSH and come in to recognize mismatched dna and initiate repair
what are the 4 steps of MMR?
1) mismatch recognition
2) recruitment of additional MMR factors
3) search for signal identifying the incorrect (newly synthesized) strand and degradation of this before the mismatch
4) resynthesis of excised dna
describe NHEJ mechanism
corrects double strand breaks (DSB)
two broken ends are joined regardless of sequence
more error prone (loss/gain of dna)
describe HR mechanism
corrects double strand breaks
similar to HR in meiosis
- needs the presence of sister chromatids
- single strand of dna from donor sister chromatid invades the damaged sister chromatid
what is dna damage response?
dna damage is detected
arrest of cell cycle
- dna tolerance or damage repair
what are some common themes of dna repair?
detect the damage
- proteins detect and bind
remove the damage
- nucleases
resynthesis/repair
- dna polymerase + ligases
regulation
- protein kinases
what are some effects/outcomes of repair?
accurate repair = survival
failure of repair = cell death
misrepair = mutation
