DNA damage and repair Flashcards
common reaction of bases with main example
amino groups of bases are reactive- the NH2 group in cytosine can be removed to convert into uracil
other reaction of bases with example and cause
bases can also be chemically modified by oxidation reactions eg thymine oxidation to thymine glycols eg due to radiation, alkylation, and adduct formation
define adduct
type of chemical modification, where larger molecules added to a base
photodamage
UV light hits two thymidines, which react with each other to form DIMERS
DIAGRAM types of DNA damage
thymine dimers base pair mismatch NICKS where one base on one strand missing as phosphodiester bond broken, so one pair affected- done by xrays/radiation GAPS are multiple nicks together, so a large part of one strand missing
causes of DNA damage
chemicals/carcinogens- dietary, lifestyle eg smoking, endogenous, radiation- solar/ionising
DNA damage by carcinogens
base dimers double/single strand breaks (gaps/nicks) base hydroxylation (base destroyed) DNA adducts/alkylation
feature of carcinogens
they are polycyclic hydrocarbons (multiple carbon rings)
metabolism (phase 1/2) overview
functional group added eg through oxidation by cytochrome P450, which allows conjugation of functional group to make water soluble ie excreted`
benzo (a)pyrene epoxidation
aka b(a)p is itself not carcinogenic, but through liver metabolism via p45O, oxidation occurs (double bond exploited) to form an EPOXIDE, which forms a DNA adduct (ie metabolite that liver produces attaches to bases)
aflatoxin B1 epoxidation
p450 converts it into an epoxide, which forms a DNA adduct with guanine= liver tumours
2-napthylamine metabolism
again not a carcinogen, which after glucoronidation, once it gets into urine, the pH of urine forms a reactive nitrenium ion, which react with DNA= bladder tumours
effect of UV radiation
causes thymine dimers= skin cancer
effect of ionising radiation
forms oxygen free radicals, which are electrophilic, so look for electron rich DNA this can lead to base modifications, and DNA strand breaks
rates of damage vs repair
max repair rate is always much higher than rate of DNA damage
role of p53
when there are double strand breaks/problems with DNA replication, inactive MDM2 forms the active TMS p53, which causes DNA repair
types of DNA repair
direct reversal of DNA damage excision base repair (for apurinic/apyrimidinic damage ie loss of purine/pyramidien) nucleotide exicision repair (for mainly large DNA adduct) during/post-replication repair
direct reversal of DNA damage- examples
involves enzyme photolysases remove thymine dimers MGMT’s enzyme/ methyltransferases reverse alkylation adducts by removing methyl group
DNA mismatch repair
occurs DURING replication polymerase adds wrong base, so proof reading ensures nuclease replaces it with correct base
DIAGRAM excision base repair with enzymes involved
mutagen causes damaged base- body responds with glycosylase, which remove base (apurine/apyrimidinic situation- NOT large DNA adduct) endonuclease cuts open DNA, polymerase adds DNA, and ligase seals it
DIAGRAM nucleotide excision repair
mutagen damages base and surrounding area, so endonuclease and helicase remove large patch of DNA, and polymerase adds correct patch
human genetic diseases involving deficiency of NER
xeroderma pigmentosum- severe sensitivity to UV light= high risk of skin cancer and other cancers
DNA double strand break repair
doube strand breaks are made, and homologous recombination occurs- another type of during/post replication repair, but more SEVERE
consequence of DNA damage
if repair doesn’t occur/incorrect, DNA replication of a different primary sequence gives defective proteins/ cancer if oncogenes/TMS affected or apoptosis
what therapeutic agents do with types
they cause DNA damage of cells to force apoptosis alkylating agents agents making large adducts eg cisplatin agents causing double strand breaks eg radiotherapy ie ionising radiation
Ames test
tests for DNA damage bacteria eg salmonella that don’t produce HISTIDINE due to mutation added to potential carcinogenic substance if substance is carcinogenic, it will correct bacterial mutation, so they can produce histidine and grow in abundance in cultures occurs in presence of liver s9 (has enzymes needed for growing)
detecting DNA damage through chromosome damage
in presence of liver S9, potential carcinogen added to chromosomes, and any damage identified
invitro micronucleus assay
cells treated with potential carcinogen in culture, and stain added to look for micronuclei (DNA budded off from normal nuclei)
bone marrow micronucleus assay
rats treated with carcinogen, and you look at erythrocytes for potential micronuclei (shouldn’t have any)
