Mod8 - DNA Mutation and Repair

Question 1

What are base tautomers?

Answer 1

Isomers with slightly different chemical structures to the normal bases

Question 2

Describe the tautomeric shift seen in adenine (full name of normal base, tautomer, and pairing change)

Answer 2

Amino-adenine -> Imino-adenine

Now pairs with C, not T

Question 3

Describe the tautomeric shift seen in guanine (full name of normal base, tautomer, and pairing change)

Answer 3

Keto-guanine-> Enol-guanine

Now pairs with T, not C

Question 4

Describe the tautomeric shift seen in cystosine (full name of normal base, tautomer, and pairing change)

Answer 4

Amino-cytosine -> Imino-cytosine

Now pairs with A, not G

Question 5

Describe the tautomeric shift seen in thymine (full name of normal base, tautomer, and pairing change)

Answer 5

Keto-thymine -> Enol-thymine

Now pairs with G, not A

Question 6

Name the key Base Analogue (and which base is it an analogue of?)

Answer 6

5-Bromo Uracil (Analogue of Thymine)

Question 7

What change does 5-Bromo Uracil sometimes undergo once incorporated into DNA that causes an issue?

Answer 7

It shifts from Keto-5bU (pairs with Adenine) to Enol-5bU (Pairs with Guanine)

Question 8

Give two examples of deaminating agents

Answer 8

Nitrous acid and sodium bisulphite

Question 9

What is Adenine deaminated to, and how does this affect its pairing?

Answer 9

Hypoxanthine (pairs with C, not T)

Question 10

What is Guanine deaminated to, and how does this affect its pairing?

Answer 10

Xanthine (blocks DNA replication)

Question 11

What is Cytosine deaminated to, and how does this affect its pairing?

Answer 11

Uracil (pairs with A, not G)

Question 12

What is Thymine deaminated to, and how does this affect its pairing?

Answer 12

Trick Question - thymine has no amine group and thus cannot be deaminated

Question 13

Name one alkylating agent and the type of mutation it causes

Answer 13

Ethylmethane Sulphonate (EMS) - causes a transition mutation

Question 14

Name one intercalating agent

Answer 14

Ethidium bromide

Question 15

What structural change does UV radiation cause in DNA (and what are the two types of this)?

Answer 15

Base dimerisation (Cyclobutyl dimers and 6-4 photoproducts)

Question 16

What structural change in DNA can be caused by heat?

Answer 16

Hydrolysis of a ß-N glycosidic bond, resulting in an AP (apurinic/apyrymidinic) site

Question 17

What enzyme can repair base dimerisations caused by UV (and in which species is it found?)

Answer 17

DNA Photolyase (E. coli)

Question 18

Define Direct Repair

Answer 18

When an enzyme corrects a nucleotide alteration caused by a mutagen

Question 19

Name one enzyme in E. coli and one in Humans which can reverse the effects of alkylating agents

Answer 19

E. coli: ADA (position 4 of T or 6 of G)
Humans: MGMT (position 6 of G)

Question 20

Which enzyme carries out base excision in E. coli?

Answer 20

DNA Glycosylase

Question 21

Which enzyme carries out nucleotide excision in E. coli (and which two parts of the enzyme actually cut the DNA)?

Answer 21

UvrABC Endonuclease (UvrB and UvrC)

Question 22

In mismatch repair, how is the (correct) parent strand identified?

Answer 22

The parent strand is methylated, whereas the daughter strand is not yet

Question 23

Which two enzymes are involved in mismatch repair (and which one actually cuts the DNA)?

Answer 23

MutS and MutH (MutH cuts the DNA)

Question 24

Which proteins attract each other in non-homologous end-joining (and which enzyme actually joins the ends together)?

Answer 24

Ku proteins (and DNA Ligase)

Question 25

In non-homologous end-joining, why are the Natural Ends of the chromosomes not joined together?

Answer 25

The telomeres mark the natural ends