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Flashcards in G2 Deck (32):

What is the luria and delbruck fluctuation testq

showed that survival of resitant colonies of of E.coli in presence of T1 phage was not because phage was inducing mutations. mutations occured at different times in different plates
phage was environmental selecting agent


what is replica plating?

lederberg - exact replica due to stamp on arg+ and arg- identifies arg mutants.


What doe mutation hotspots on lacl show?

1. that there is a 5'methylcystosine at each hotspot which can be deaminated to thymine
2. that CTGG is a major indel hot spot, repeated tandemly - addition or loss leads to mutation


what is depurination and rate in mammals

loss of pruine base
mammalian cells can lose 1-^4 per 20 hour gen.


what is replication slippage

section newly synthesized loops back on itself in ssloop DNAP copies region second time due to loop and in the enxt generation therefore get one with normal one with more repeats.
usually happens at repeats `


What are most liable to cross over unequallyy

cross at ares of homology


how are photodimers created by UV fixed?

Repaired by CPD photolyase which binds to and split dimers = photoreactivation
(cyclobutane photodimer = CPD)


how is O-6 guinanine alkylation repaired?

alkytransferases remove groups
eg. methyltransferase AlkB family of deoxygenases.


how do DNA glycosylases work?

hydroylse sugar bonds releasing the altered base leaveing an abasis site


how does AP endonuclease work

cleaves damaged strand at 5drp of abasic site


how does AP beta lyase work

cleaves DNA at 3'drp of abasic site


how does AP beta delta lyase worjk

cleaves at 5' adn 3' phosphates around abasic site


how are holes left by removed abasic sites fixed

DNAP replaces excised
XRCC1 ligase alpha seals nick


Describe long patch BER

initiated by AP endonuclease - eg. APEX1 which cleaves DNA 5' to abasic site, leaving 3' hydroxyl group as substrate for DNAP beta
which fills removed base and several bases downstream displacing the strand on the 3' side of cut.
displaced strand is removed by flap endonuclease and ligated by DNA ligase.


what does transcription coupled nt excision repair do?

repairs transcribed DNA regions


what does global genomic repair GGR do

correct lesions in any section of genome activated by stalled replication forks


describe tc NER

when transcription by RNAP stalled by lesions then CSA and CSB bind to complex
attracting TF11H (10 subunits) which contain helicases XPB and XPD
miscoding region cut out and replaced


describe GCR

damage sensor XPC scans for distorted DNA, recognises it and binds to opposite strand
XPC attracts TF11H via XPB ATPase activity.


1. what does TF11H do?

opens DNA allowing recuritment of XPA and CDK activating kinases (CAK)


2. what does XPA do?

allow release of TF11H and CAK and recruitment of RPA (XPD heicase activity is relased open the DNA further. XPC-CEN2 is released from the complex)


3. what does RPA do?

coats the undamaged DNA strands


4. what does the release of CAK faciliate?

recruitment of XPF-excision repair complementeing (ERCC1) complex and XPG


5. what does XPF excision repair do?

carry out incision 5' and 3' in the damged DNA and DNAP fills gap. and XRCC1 ligates


Describe the process of DNA mismatch repair

1. reconigtion of damage in enwly replicated DNA by MutS
2.Binding of MutS to distrotions in DNA cuased by mismatches
- it recognises parent strand and nicks daughter strand
3. attracts MutL, MutH and UVrD
4.MutH cuts the section with the incorrect bases and UVrD DNA helicase activity unwinds DNA.
5. Error containing strand is degraded by ssDNA specific exonucleases (e.g Exo1, ExoV11) and resynthesized by DNAP


what non homolgous end joining NHEJ

major pathway for repairing ds breaks DSBR


Describe the process of NHEJ

1. initiated by binding of Ku70 and Ku80 to DSBR
2. this recruits DNAPKcs and aligns DNA ends
3. DNAPK recruits endonuclease Artemis which trims the 5' and 3' overhangs
4. PNKP is recruited via XRCC4 - LIG4 releases exisiting 5'p and 3'p residues
5. XLF interacts with XRCC4 - LIG4 and DNAP


Describe homologous recombination

recognition of double stranded break by MRN complex
MRN associated CtBP which produces 3 ssDNA overhangs and 3ss DNA is stabilised by RPA
RAD52 promotes replacement of RPA with RAD51 to allow fromation of RD51-DNA nuceloprotein filaments
BRAC2 assists the assembly of RAD51 onto RPA ssDNA, RAD51 coated ssDNA enables strand invasion.


what is the first step in all BER reactions

removal of abasic site by dna glysoylase


what does TFIIH contain

10 subunits including XPB and XPD which are helicases


what organisms use BER

pro and euk


what are two diseases caused by defects in excision repair systems and how do they differ

xeroderma pigemetosum - has XPG defects
Cockayne syndrome = CSA and CSB defects


what does cockayne syndrome cause

dwarfism, deafness and retardation plus premature aging