DNA damage and repair Flashcards
What can damage DNA?
Chemicals - dietary (40%), medical, lifestyle, environmental, occupational, endogenous
Radiation - ionising, solar, cosmic
What kind of damage can occur to DNA?
> base dimers and chemical cross links
DNA adducts and alkylation)
base hydroxylation and abasic sites
double/single strand breaks
What is deamination?
primary amino groups converted to ketogroups
What are chemical modifications?
Several types of hyper reactive oxygen species can modify DNA bases
What is photodamage
UV light absorbed by nuclei acid bases causes influx of energy which can induce chemical changes. - most commonly bond formation between adjacent pyrimidines within one strand
Where do polycyclic aromatic hydrocarbons (Benzo[a]pyrenes/B[a]Ps) originate from?
Common environmental pollutants
Formed from combustion of fossil fuels
Formed from combustion of tobacco
What happens in the first phase of mammalian metabolism and where does it occur
> Phase 1 – addition of functional groups – oxidations, reductions, hydrolysis.
o Mediated mainly by cytochrome p450 enzymes.
What happens in the second phase of mammalian metabolism and where does it occur
Phase 2 – conjugation of phase 1 functional groups – glucuronidation, sulphation, glutathione conjugation, methylation, acetylation & amino-acid conjugation.
o Generates polar (water soluble) metabolites to excrete.
How is B[a]P epoxidated?
B[a]P epoxidation process:
o P450 enzymes oxidise the B[a]P (becomes very reactive).
o EH (epoxide hydrolase) removes the toxic oxide.
o P450 again oxidises the B[a]P which then degrades spontaneously.
o +ve-charged B[a]P then adducts onto DNA.
How is aflatoxin B1 epoxidated?
Aflatoxin B1 epoxidation process:
o P450 oxidises the aflatoxin B1.
o Aflatoxin B1 the adducts to DNA directly using its adjacent N7 positively charged carbon atom.
What are some sources of aflatoxin B1
o Formed by Aspergillus flavus mould and is commonly found in poorly stored grains and peanuts.
o Is a potent liver carcinogen (in Africa/far-east).
What is 2-naphthylamine and where does it come from?
2-naphthylamine:
o A past component of dye-stuffs and includes benzidine.
o 2-nap is a potent human bladder carcinogen.
How is 2-naphthylamine metabolised and cause DNA damage?
2-nap metabolism:
o Cytochrome P1A2 oxidises the amine group.
o Glucuronyl transferase adds a glucuronide group to the amine which is the broken by the acidic urine pH.
o The nitrenium ion remaining then causes DNA damage in the bladder.
How does solar (UV) radiation cause DNA damage?
o UV radiation stimulates formation of pyrimidine (thymine) dimerisation skin cancer.
How does ionising radiation cause DNA damage>?
Ionising radiation:
o Generates free radicals inside cells such as oxygen free radicals – super oxide (O2•) and hydroxyl (HO•).
o These oxygen free radicals have unpaired electrons that are electrophilic and so seek out electron-rich DNA (negatively charged sugar-phosphate backbone of DNA).
What kind of damage do free radicals do to DNA>
o DNA damage from oxygen free radicals includes:
Double/single stand breaks.
Apurinic & apyrimidinic sites – sites where the base is lost whilst the backbone remains.
Base modifications:
• Ring-opening – guanine & adenine.
• Glycol (unstable products of oxidation) formation – thymine & cytosine.
• Creation of 8-hydroxyadenine & 8-hydroxyguanine – mutagenic.
How does p53 deal with cellular stress?
p53 can sense the DNA damage and regulate the response of the cell to this event.
Describe direct reversal of DNA repair
o Photolyase splits cyclobutane pyrimidine dimers.
o Methyltransferases & alkyltransferases remove alkyl groups from the bases.
Base excision repair – mainly for apurinic & apyrimidinic damage:
o DNA glycosylases & apurinic/apyrimidinic endonucleases.
o A repair polymerase (e.g. Pol-beta) fills the gap and DNA ligase completes the repair.
Nucleotide excision repair – mainly for bulky DNA adducts:
o Xeroderma pigmentosum proteins (XP proteins) assemble at damage stretch of nucleotides are excised.
o A repair polymerase (e.g. Pol-beta) fills the gap and DNA ligase completes the repair.
During- or post-replication repair:
o Mismatch repair. - scrutinisation of DNA for a-posed bases that do not pair properly
o Re-combinational repair.
What is the pathway in base excision repair?
o DNA-glycosylase – removes the base.
o AP-endonuclease – cuts the backbone.
o DNA polymerase – adds a complimentary base.
o DNA ligase – re-joins the backbone.
What is the pathway in nucleotide excision repair?
o Endonuclease – cuts a large swathe of backbone.
o Helicase – removes 1+ nucleotides.
o DNA polymerase – adds complimentary bases.
o DNA ligase – re-joins the backbone.
What is the pathway in nucleotide excision repair?
o Endonuclease – cuts a large swathe of backbone.
o Helicase – removes 1+ nucleotides.
o DNA polymerase – adds complimentary bases.
o DNA ligase – re-joins the backbone.
