DNA damage Flashcards
How does DNA pol reduce the probability of incorrect base pairing
Base selection and proofreading - If the wrong base is added during DNA rep, DNA pol moves back, acting as an exonuclease. It degrades the recently synthesised strand and then proceeds once the right base has been put in place.
Which DNA pol is responsible for the bulk of leading and lagging strand synthesis? What is the result of a D400A mutant in mice>
DNA pol delta - Mice die due to malignancies.
Which side of the equilibrium of G/T structure is most favoured? Keto or Enol
Keto
What is the result of T being present in its enol form at replication
It can form 3 hydrogen bonds with G, causing a transition mutation
Which side of the equilibrium of C/A structure is most favoured? Amine or Imine
Amine
What is the result of A being present in its Imine form at replication?
May form 2 hydrogen bonds with C, causing a transition mutation
Which bases are pyrimidines
C and T
Which bases are purines
G and A
What is the pairing rule as a result of a tautomeric shift
Modified purine bases pair with the “wrong” pyrimidine (Enol G with Keto T, Imine A with Amine C)
Modified pyrimidine pairs with the “wrong” purine (Enol T with Keto G, Imine C with amine A)
Which two deamination events are mutagenic?
Cytosine to Uracil and Adenine to hypo-xanthine
How does depurination occur
The B-N glycosidic bond is hydrolytically cleaved, releasing guanine or adenine (purines) from the DNA strand.
Which is more common, depurination or depyrimidination
Depurination
Give the two types of environmental DNA damage
Chemical agents eg alkylating agents and aflatoxins
Physical agents eg UV light and ionising radiation
What are the two main sites of alkylation and why is this the case?
Guanine-N7 major groove and adenine-N3 minor groove. Because these are the most negatively charged positions on DNA bases, attracting the positively charged alkyl groups.
Is the 7-alkyl-guanine adduct a mutagenic lesion?
No - glycosidic bond slowly hydrolyses to give apurinic site which is repaired.
Is 3-alkyl-adenine adduct a mutagenic lesion
Yes - as it sits in the minor groove, this prevents the progression of DNA pol. Prevents DNA replication and transcription as a result.
What is the result of the 06-alkyl-guanine adduct
Locks guanine in its enol form. This can therefore form hydrogen bonds with either C or T, if T it will cause a G to A transition.
How important is a G to A transition in the initiation of cancer?
Single G to A transition is responsible for the oncogenic switch in HaRas gene.
How would the 3-alkyladenine, 7-alkylguanine and O6-alkyl-guanines best be described?
Monofunctional
What form of bifunctional agent is most dangerous to a cell
Agents capable of producing interstrand crosslinks as they prevent the unwinding of DNA by creating covalent bonds between the two strands of DNA. This prevents both DNA replication and transcription
What is Aflatoxin B1 and what is the cancer that it is associated with
It is a fungus that grows on nuts in humid conditions. Cyt P450 metabolises the fungus, one of the metabolites is able to adduct to DNA. This adduct is highly distorting of DNA and is associated with hepatocellular carcinoma.
What is Benzopyrene and how does it initiate cancer
Chemical found in coal tar. One route of metabolism produces benzo-[a]-pyrene, a DNA adduct. This is highly distorting of DNA and can produce a C to T transition
How does UV light induce DNA damage?
The max wavelength of light that skin can absorb is 260nm (the middle of the UV light spectrum). Once absorbed, an energy transfer forms a cyclobutyl ring between pyrimidine bases which produces kinks in the DNA.
Which pyrimidine bases are cyclobutyl rings most likely to form between, and as a result of what agent?
T-T, as a result of UV light
What is Psoralen
A photosensitization agent
How does Psoralen aid in the damage of DNA
It’s a flat planar molecule which can intercalate between DNA bases. It is also highly sensitive to UV light. When subjected to UV it forms a covalent interstrand crosslink.
What are the direct ways in which ionising radiation induces DNA damage
SSBs, DSBs, base oxidation, DNA-protein crosslinks
What is the indirect way in which DNA damage is caused by ionising radiation
Radical formation
What changes in base are caused by deamination events?
Cytosine - uracil
Adenine - hypo-xanthine
Guanine - Xanthine
5-methylcytosine - Thymine
