Lecture 5 Flashcards
How do chemical mutagens work?
- Work by modifying base-pairing properties
- DNA polymerase then incorporates wrong bases during DNA replication
Describe what happens with EMS
- 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
How have the pairing properties of Guanine been changed following binding of the ethyl group?
The pairing properties change so now Guanine pairs to thymine
What kind of mutation does EMS cause?
- It is an alkylating agent
- Causes GC -> AT mutation
What is the effect of 2-amino purine?
- 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
What is aflotoxin?
- Produced by fungi
- Chemically reacts with guanine and causes G-C -> T-A mutation
What does Aflotoxin B do?
Reacts with DNA and creates APURINIC SITES by destabilising the bond that holds a base to DNA backbone (can cause breakage)
What is depurination?
A purine base (A/G) is lost
Following depurination and during replication, what purine base is most likely to be inserted?
Adenine
Can DNA pol read past DNA lesions?
No
Describe translesion DNA synthesis
Bypass DNA read past the DNA lesions however are likely to incorporate wrong base even when DNA is not damaged
What do all mutation mechanisms require to become mutations?
DNA replication
Where does DNA repair occur?
At the pre-mutagenic lesion
In reference to DNA replication when do DNA repair systems fix DNA?
BEFORE replication
How is EMS damage repaired?
Repair proteins -alkyltransferases- remove EMS group and permanently bind onto itself.
Guanine is now normal
How are Apurinic sites repaired?
Through process of Nucleotide excision repair
- APendonuclease recognises the AP site and cuts the strand containing the site
- Defective DNA and some adjacent DNA is removed by excision exonuclease
- Gap is filled with DNA synthesis
How does radiation (UV) cause mutation?
- 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
What are the two mechanisms to photodimer repair?
- Photolyase enzyme - uses energy from white light to convert photodimers back to pyrimidines (photo repair)
- Nucleotide excision repair - similar to AP sites
Bacteria used in the Ames test lack a protein needed to do a repair process, what process is this?
Nucleotide excision repair
What does colibacterin do?
A compound secreted by some E.coli bacteria that can live in the human gut
-Related to colorectal cancer
