DNA Damage and Repair

Question 1

what is a mutation?

Answer 1

any change from the normal dna sequence

Question 2

what are types of mutations?

Answer 2

deletions
substitutions
insertions
transitions
transversions

Question 3

what is a deletion mutation?

Answer 3

loss of nucleotide

Question 4

what is an insertion mutation?

Answer 4

gain of nucelotides

Question 5

what is a substitution mutation?

Answer 5

change of one base for another

Question 6

what is a transition nucleotide?

Answer 6

A to G (purine to purine)
C to T (pyrimidine to pyrimidine)

Question 7

what is a transversion mutation?

Answer 7

purine to pyrimidine

Question 8

what are some examples of exogenous (external) damage?

Answer 8

ionising radiation (x rays)
UV (sunlight)
chemicals
base modification and cross-linking between bases

Question 9

what are some examples of endogenous (internal) damage?

Answer 9

dna replication errors
hydrolysis - water drives the reaction
oxidation

Question 10

describe hydrolysis: deamination

Answer 10

loss of an amino group
cytosine to uracil
adenine to hypoxanthine
5 methylcytosine to thymine

Question 11

describe hydrolysis: depurination

Answer 11

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

Question 12

what are the consequences of dna damage?

Answer 12

cell death
functional decline of tissues
organ issues/failure
cancer
developmental deficiencies
embryonic lethality

Question 13

what are the 7 dna repair mechanisms?

Answer 13

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)

Question 14

describe direct repair mechanism

Answer 14

infrequently used, 3 genes implicated
O6-methylguanine - dna methyltransferase, removes methyl grps

Question 15

describe BER mechanism

Answer 15

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

Question 16

describe NER mechanism

Answer 16

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

Question 17

describe MMR mechanism in simple terms

Answer 17

correct the mismatched base pairs or small insertion/deletion loops caused by errors in dna replication

Question 18

what is the function and use of MSH proteins?

Answer 18

form ‘sliding clamps’ that move along the dna to identify damage

targeted by MSH and come in to recognize mismatched dna and initiate repair

Question 19

what are the 4 steps of MMR?

Answer 19

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

Question 20

describe NHEJ mechanism

Answer 20

corrects double strand breaks (DSB)
two broken ends are joined regardless of sequence
more error prone (loss/gain of dna)

Question 21

describe HR mechanism

Answer 21

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

Question 22

what is dna damage response?

Answer 22

dna damage is detected
arrest of cell cycle
- dna tolerance or damage repair

Question 23

what are some common themes of dna repair?

Answer 23

detect the damage
- proteins detect and bind
remove the damage
- nucleases
resynthesis/repair
- dna polymerase + ligases
regulation
- protein kinases

Question 24

what are some effects/outcomes of repair?

Answer 24

accurate repair = survival
failure of repair = cell death
misrepair = mutation

Question 25

what would happen to each repair mechanism of it was defective?

Answer 25

BER = colon cancer
NER = UV sensitivity, xenoderma pigmentosa
MMR = Heredity nonpolyposis colon cancer
HR = bloom syndrome
NHEJ = leukaemia
DNA damage response and repair = BRCA1

Question 26

describe BRCA1 breast cancer

Answer 26

involved in both checkpoint arrest and double stranded dna repair

females w/ abnormal gene have 80% risks of developing brac1 at age 90