DNA Repair

Question 1

What is cellular balance?

Answer 1

Balance between the avoidance of mutations that affect genetic stability and the generation of mutations that do not affect genetic stability and thus allow for selection of genetic fitness

Question 2

What are the 5 responses to DNA damage?

Answer 2

Cell -cycle checkpoint activation 
Transcriptional activation
Apoptosis 
Damage tolerance 
DNA repair

Question 3

What are the 3 types of DNA Repair?

Answer 3

Reversal of base damage
Excision of base damage
Repair of double strand breaks

Question 4

What is the simplest mechanism of DNA damage repair

Answer 4

Reversal of base damage

Question 5

What does reversal of base damage involve?

Answer 5

Single enzyme composed of a single polypeptide chain that catalysed a single ate reaction

Question 6

What is photoreactivation?

Answer 6

DNA photo lyase repairs pyrimidine dimers formed by UV radiation exposure

Question 7

What is significant about DNA photolyase?

Answer 7

The gene for this enzyme is lost in all mammals

Question 8

What does the enzyme O 6 methylguanine DNA methyl transferase do?

Answer 8

Transfers methyl group of O6 methylguanine to a cysteine residue on the enzyme

Question 9

Why is O6 methylguanine DNA methyl transferase important?

Answer 9

Important in defence against alkylating agents

Question 10

What are the 3 mechanisms of excision of base damage?

Answer 10

Mismatch excision repair
Nucleotide excision repair
Base excision repair

Question 11

What does excision repair involve?

Answer 11

A physical removal and replacement of damaged bases

Question 12

What does MMR involve ?

Answer 12

Excision of nucleotides that are incorrectly paired with the correct nucleotide on the opposite DNA strand

Question 13

During MMR small loops that are generated by the insertion / deletion of nucleotides are recognised by what?

Answer 13

MUTS alpha complex

Question 14

MUTS alpha is a heterodimer of what?

Answer 14

MSH2 and MSH6

Question 15

Recognition of small loops can also be carried out by an alternative Heterdimer, what?

Answer 15

MUTS beta (MSH2 and MSH3)

Question 16

The following steps after recognition of small loops during MMR are unknown, by the proteins involved are know. What are they?

Answer 16

MLH1,PMS2, MLH3

Question 17

Defects in MMR are prominent in what cancer?

Answer 17

Colon

Also uterine, ovarian, gastric

Question 18

What does nucleotide excision repair involve?

Answer 18

Excision of approx 30 bases around the damaged site

Question 19

There are 3 stages of NER, what are they?

Answer 19

Recognition of base damage
Bimodal incision
Repair synthesis and ligation

Question 20

Describe the recognition of base damage in NER?

Answer 20

XPC bind HHRAD23B

heterodimer formation leads to the recruitment of XPA, RPA (both recognise damaged DNA ) TF11H (two subunits XPB AND XPD - helicases) , and XPG

Now ERCC1 and XPF bind and complete repairosome formation

Question 21

Describe the process of bimodal incision in NER?

Answer 21

XPG - endonuclease -makes a 3’ incision 3-5nt from damaged site

ERCC1-XPF makes a 5’ incision 24-25nt from damaged site

27-30nt excision fragment generated

Question 22

Describe repair synthesis during NER?

Answer 22

RFC loads PCNA trimeric circle on DNA by dissociating the post incision complex

Facilitating PCNA association with DNA pol e or D

Gap filled and repair patch ligated by ligase

Question 23

NER defects are associated with what type of cancer?

Answer 23

Skin

Question 24

What is base excision repair?

Answer 24

Excision of a single base

Question 25

How is BER initiated?

Answer 25

DNA-glycosylases

Question 26

How do DNA glycosylases initiate BER?

Answer 26

Recognise a single / small subset of altered bases and catalyse the hydrolysis of the N-glycosidic bond that links base to the sugar-phosphate backbone

Question 27

During BER what is generated?

Answer 27

Apurinic/apyrimidinic sites

Question 28

Apurinic/apyrimidinic sites are recognised by what?

Answer 28

AP endonuclease

Question 29

What does AP endonuclease do?

Answer 29

Cuts DNA phosphodiester backbone at 5' end

Question 30

After AP endonuclease has produced a 5' nick what is the subsequent step during BER?

Answer 30

DNA deoxyribophosphodiesterases remove the entire nucleotide

Question 31

What are the two mechanisms that repair double strand breaks?

Answer 31

Homologous recombination | Non homologous end joining

Question 32

What is homologous recombination?

Answer 32

Damaged sites repaired using the information on the Undamaged homologous region on the other chromosome

Question 33

Deficiencies in homologous recombination are implicated in what cancer?

Answer 33

Breast - BRCA1 genes

Question 34

What mechanism is DNA damage tolerated?

Answer 34

Translesion DNA synthesis

Question 35

What is translesion DNA synthesis?

Answer 35

Replication machinery bypass sites of base damage due to sloppy DNA pol's