DNA damage and repair

Question 1

common reaction of bases with main example

Answer 1

amino groups of bases are reactive- the NH2 group in cytosine can be removed to convert into uracil

Question 2

other reaction of bases with example and cause

Answer 2

bases can also be chemically modified by oxidation reactions eg thymine oxidation to thymine glycols eg due to radiation, alkylation, and adduct formation

Question 3

define adduct

Answer 3

type of chemical modification, where larger molecules added to a base

Question 4

photodamage

Answer 4

UV light hits two thymidines, which react with each other to form DIMERS

Question 5

DIAGRAM types of DNA damage

Answer 5

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

Question 6

causes of DNA damage

Answer 6

chemicals/carcinogens- dietary, lifestyle eg smoking, endogenous, radiation- solar/ionising

Question 7

DNA damage by carcinogens

Answer 7

base dimers double/single strand breaks (gaps/nicks) base hydroxylation (base destroyed) DNA adducts/alkylation

Question 8

feature of carcinogens

Answer 8

they are polycyclic hydrocarbons (multiple carbon rings)

Question 9

metabolism (phase 1/2) overview

Answer 9

functional group added eg through oxidation by cytochrome P450, which allows conjugation of functional group to make water soluble ie excreted`

Question 10

benzo (a)pyrene epoxidation

Answer 10

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)

Question 11

aflatoxin B1 epoxidation

Answer 11

p450 converts it into an epoxide, which forms a DNA adduct with guanine= liver tumours

Question 12

2-napthylamine metabolism

Answer 12

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

Question 13

effect of UV radiation

Answer 13

causes thymine dimers= skin cancer

Question 14

effect of ionising radiation

Answer 14

forms oxygen free radicals, which are electrophilic, so look for electron rich DNA this can lead to base modifications, and DNA strand breaks

Question 15

rates of damage vs repair

Answer 15

max repair rate is always much higher than rate of DNA damage

Question 16

role of p53

Answer 16

when there are double strand breaks/problems with DNA replication, inactive MDM2 forms the active TMS p53, which causes DNA repair

Question 17

types of DNA repair

Answer 17

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

Question 18

direct reversal of DNA damage- examples

Answer 18

involves enzyme photolysases remove thymine dimers MGMT’s enzyme/ methyltransferases reverse alkylation adducts by removing methyl group

Question 19

DNA mismatch repair

Answer 19

occurs DURING replication polymerase adds wrong base, so proof reading ensures nuclease replaces it with correct base

Question 20

DIAGRAM excision base repair with enzymes involved

Answer 20

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

Question 21

DIAGRAM nucleotide excision repair

Answer 21

mutagen damages base and surrounding area, so endonuclease and helicase remove large patch of DNA, and polymerase adds correct patch

Question 22

human genetic diseases involving deficiency of NER

Answer 22

xeroderma pigmentosum- severe sensitivity to UV light= high risk of skin cancer and other cancers

Question 23

DNA double strand break repair

Answer 23

doube strand breaks are made, and homologous recombination occurs- another type of during/post replication repair, but more SEVERE

Question 24

consequence of DNA damage

Answer 24

if repair doesn’t occur/incorrect, DNA replication of a different primary sequence gives defective proteins/ cancer if oncogenes/TMS affected or apoptosis

Question 25

what therapeutic agents do with types

Answer 25

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

Question 26

Ames test

Answer 26

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)

Question 27

detecting DNA damage through chromosome damage

Answer 27

in presence of liver S9, potential carcinogen added to chromosomes, and any damage identified

Question 28

invitro micronucleus assay

Answer 28

cells treated with potential carcinogen in culture, and stain added to look for micronuclei (DNA budded off from normal nuclei)

Question 29

bone marrow micronucleus assay

Answer 29

rats treated with carcinogen, and you look at erythrocytes for potential micronuclei (shouldn’t have any)