Mutations and DNA Repair Flashcards
Deamination of adenine results in ____________ which base pairs with ________________.
Hypoxanthine, cytosine
Causes of DNA mutation
- Mistakes made in DNA replication
2. Mutations made by chemical and physical damage to DNA
- Mutations made by chemical and physical damage to DNA
- Hydrolysis= a basic site
- Deamination by HNO2
- Deamination by H2O
- Oxidative damage
- Alkylation of bases
- Cross linking
- Strand damage
- Intercalating agents
Cytosine deaminated to
Uracil
3 mechanisms of deamination
Removal of NH2
- Hydrolysis
- Hydrolysis=> abasic site
- Nitrous acid HNO2
Sodium disulfite deaminated 5-methylcytosine to
Thymine
T or F
Sodium disulfite deaminates cytosine
Therefore
False
Sodium disulfite does NOT deaminate cytosine
Therefore sodium disulfite can be used to gauge amount of methylated DNA
Guanine deaminated to
Xanthine (still base pairs with cytosine, but only with 2 H-bonds instead of 3)
Are purines or pyrimidines more susceptible to abasic site formation by hydrolysis?
Purines. AG
Oxidative damage is caused by
- ROS
- Electron transport chain ETC
- Metabolic products
Most common deamination by hydrolysis
C to U
Oxidative damage: 8-oxoG is mutagenic because
8-oxoG anti bps with C anti (3 H bonds )
Normal relative to G anti bps with C anti (3H bonds)
8-oxoG(syn)z=A(anti)
Mutagenic relative to
T(anti)=A(anti) /
G(anti) 3 H-bonds C(anti)
- Mistakes made in DNA replication
- Proofreading error = transition/ transversion
- Indel= frameshift
- DNA microsatellites= polymorphisms, polyQ
Oxidative damage: _______ is mutagenic
8-oxoG
Alkylation of bases: A,G purines
C8
Alkylation of bases: C,T pyrimidines
C5, C6
Alkylation of bases: product of
Smoke-> process C molecules in liver
Ames test
- Control- natural bg f of rev mutants by playing on min His media
- Salmonella + potential mutagen on filter paper in middle of plates (treat w rat liver enzymes)
- Large clear zone = more potent mutagen ; more colonies that grow outside clear zone = more mutagenic substance
Salmonella = His auxotrophs
Revertant mutants
Test in animal sys to determine if carcinogenic
Intercalating agents cause ______________________________ possibly bc of ___________ or ______________
Insertion / deletion mutations
DNAP ( or RNAP) slippage
Distortion to DNA structure
Damage by gamma radiation and x rays generates
- Free radicals and ROS
- dsDNA breaks that prevent replication
- replication for stalls, collapses
X rays are more severe than
Solar radiation
Damage by gamma radiation and x rays: nick in 1 strand fragments how many chromosomes?
1
Damage by gamma radiation and x rays: nick in 2 strands fragments how many daughter chromosomes?
2/ both
Cross linking by solar radiation: UV light =
- Cyclobutane thymine dimers
2 covalent bonds
(Affects adjacent pyrimidines)
(No H-bonding with complementary strand) - 6,4- Photoproduct
1 diagonal bond between C6 and C4 of adjacent thymines
Conformation change in DNA backbone stalls polymerase which most commonly falls off or goes through the thymine dimer Photoproduct = mutation
Direct repair (no base removal) of methylated bases: Single use enzyme
Methyltransferase (transfers methyl from O6-methylguanine to its cys, targeting it for degradation)
Base excision repair
Remove DNA
Occurs after replication
Functions on single damaged bases that distort DNA very little
Repair unligatable single strand nicks
BER in bacteria
- Glycosylase= remove base
- AP endonuclease= ss nick
- Pol I = 5’-> 3’ exonuclease
- Ligase seals nick
BER in eukaryotes
- Glycosylase= remove base
- AP endonuclease= ss nick
- Pol
- Flap endonuclease
- Ligase seals nick
= long patch
OR
- Pol Beta (5’-dRP lyase)
- Pol beta synthesizes 1 base
- Ligase seals nick
= short patch
