Lecture 5

Question 1

How do chemical mutagens work?

Answer 1

• Work by modifying base-pairing properties

- DNA polymerase then incorporates wrong bases during DNA replication

Question 2

Describe what happens with EMS

Answer 2

• Ethyl-methane sulphonate
• An ethyl group is donated from EMS and goes to Guanine
• It binds to the O on Guanine which is used in base pairing of G

Question 3

How have the pairing properties of Guanine been changed following binding of the ethyl group?

Answer 3

The pairing properties change so now Guanine pairs to thymine

Question 4

What kind of mutation does EMS cause?

Answer 4

• It is an alkylating agent

- Causes GC -> AT mutation

Question 5

What is the effect of 2-amino purine?

Answer 5

• It most often pairs to thymine but can also pair to cytosine
• It binds to the NH2 of adenine during replication
• If this occurs it leads to a A-T changing to A-C then to G-C

Question 6

What is aflotoxin?

Answer 6

• Produced by fungi

- Chemically reacts with guanine and causes G-C -> T-A mutation

Question 7

What does Aflotoxin B do?

Answer 7

Reacts with DNA and creates APURINIC SITES by destabilising the bond that holds a base to DNA backbone (can cause breakage)

Question 8

What is depurination?

Answer 8

A purine base (A/G) is lost

Question 9

Following depurination and during replication, what purine base is most likely to be inserted?

Answer 9

Adenine

Question 10

Can DNA pol read past DNA lesions?

Answer 10

No

Question 11

Describe translesion DNA synthesis

Answer 11

Bypass DNA read past the DNA lesions however are likely to incorporate wrong base even when DNA is not damaged

Question 12

What do all mutation mechanisms require to become mutations?

Answer 12

DNA replication

Question 13

Where does DNA repair occur?

Answer 13

At the pre-mutagenic lesion

Question 14

In reference to DNA replication when do DNA repair systems fix DNA?

Answer 14

BEFORE replication

Question 15

How is EMS damage repaired?

Answer 15

Repair proteins -alkyltransferases- remove EMS group and permanently bind onto itself.
Guanine is now normal

Question 16

How are Apurinic sites repaired?

Answer 16

Through process of Nucleotide excision repair

1. APendonuclease recognises the AP site and cuts the strand containing the site
2. Defective DNA and some adjacent DNA is removed by excision exonuclease
3. Gap is filled with DNA synthesis

Question 17

How does radiation (UV) cause mutation?

Answer 17

• Adjacent thymidine (T) bases can become covalently cross linked forming a photodimer
• These fail to correctly base pair during replication
• Translesion DNA pol can replicate past but may incorporate wrong base

Question 18

What are the two mechanisms to photodimer repair?

Answer 18

1. Photolyase enzyme - uses energy from white light to convert photodimers back to pyrimidines (photo repair)
2. Nucleotide excision repair - similar to AP sites

Question 19

Bacteria used in the Ames test lack a protein needed to do a repair process, what process is this?

Answer 19

Nucleotide excision repair

Question 20

What does colibacterin do?

Answer 20

A compound secreted by some E.coli bacteria that can live in the human gut

-Related to colorectal cancer