What shape are bases?
planar (flat)
Why are bases very reactive?
Ring structure + double bonds
Example of a base reaction that can happen very easily which we dont want to happen?
Thymine – >uracil
dont want uracil in DNA
2 things base modifications can lead to:
Prevents replication
Mutation
3 types of base modifications:
Deamination
Chemical modification
Photodamage
Define deamination:
Removal of amine from nitrogenous bases
What is released in deamination reactions?
ammonia
Example of a deamination reaction:
Cytosine –> uracil
What is the key reaction in chemical modification of bases?
oxidation
Example of chemical modification reaction:
thymine–>thymine glycol
Once oxidised what can the bases go on to do?
react with other molecules e.g. larger carcinogens to form adducts
Define adduct?
base covalently bound to much larger molecules
What is photodamage of DNA caused by?
UV
Where does Photodamage occur and what is this effect called?
within strands
=intrastrand effect
What does the UV light actually cause?
thymine dimers
3 types of DNA damage:
Base pair mismatch
Thymine dimers
Radiation
Explain base pair mismatch:
When there is a mutation on 1 strand of the DNA the other base on the other strand is no longer complementary
What is seen in the DNA structure as a result of base pair mismatch?
DNA bulges
What do thymine dimers do to DNA?
disrupt DNA helix
What does radiation cause in DNA?
breaks in phosphodiester backbone–>nicks
if alot of nicks together=gaps
2 possible causes of DNA damage with examples?
Chemical e.g. lifestyle/environment/dietary
Radiation e.g. ionising
What are chemicals that cause DNA damage called?
carcinogens
What structure do carcinogens tend to have?
polycylic aromatic hydrocarbon
-double bonds
What 2 things can DNA damage lead to?
mutations
cancer
What do anti-cancer therapies often try to do?
cause DNA damage in cancer cells
4 types of damage caused by carcinogens:
DNA adducts
Base dimers
Strand break (nicks + gaps)
Base hydroxylations–>abasic sites
What do we do to carcinogens and why is this an issue?
metabolise them
often from a non-carcinogenic form–>carcinogenic
Where is phase 1 metabolism of carcinogens and what is it carried out by?
liver
cytochrome P450 enzymes
3 examples of chemicals that become carcinogens due to our metabolism of them:
B{a}P–>lung cancer
Aflatoxin B1–>liver cancer
2-napthylamine–>bladder cancer
What does solar vs ionising radiation do to DNA?
solar=thymine dimers
Ionising=breaks in phosphodiester back bone (nicks + gaps)
Which gene is key to DNA repair?
p53
What kind of gene is p53?
tumour suppressor
What does p53 do?
transcription factor for genes involved in DNA repair
3 types of DNA damage that can stimulate p53?
mitotic apparatus dysfunction
strand breaks
DNA replication arrest
How is p53 kept inactive?
MDM2 protein
4 types of DNA repair and what each is for:
Direct reversal of DNA damage-reversal/simple removal of damage
Base excision repair-apurinic/apyrimidic damage-just 1 base
nucleotide excision repair-for bulky DNA adducts
During/post replication repair e.g. mismatch repair
Explain base excision repair:
Phosphodiester backbone is fine so want to remove base without effecting it
DNA glycosylase enzyme removes base
AP endonuclease opens up DNA where there is no base
DNA polymerase replaces base
DNA ligase
What is the difference between base excision and nucleotide excision repair?
Helicase enzyme rather than DNA glycosylase as phosphodiester backbone is also removed
When can mismatch repairs take place?
during replication
Example of 3 genetic diseases involving nucleotide excision repair:
Xeroderma igmentosum
Trichothiodystrophy
Cockaynes syndrome
How are DNA double strand breaks repaired?
DNA is chewed back further in hope that sequences that match eachother are found
What is the issue with how DNA double strand breaks are repaired?
Bits of DNA are missing
What is the alternative way DNA double strand breaks can be repaired?
Non-homologous end joining-ends forcefully held together by Ku protein
3 possible outcomes of DNA damage:
Efficient repair
Incorrect repair–>mutations
Apotosis
Apart from causing transcription of DNA repair proteins what 2 other things can p53 cause?
apoptosis
growth arrests-to allow for time for DNA repair
What do cancer treatments often do and why?
cause DNA damage so p53 causes apoptosis
How do we test if something is carcinogenic?
biological assays in bacteria–>cells–>rodent–>mammal
What is a key sign of carcinogenesis in cells?
micronuclei
