Lecture 31: DNA Repair Flashcards
transient mutations
result of: faulty DNA synth deamination of cytosine and adenine environmental damage very rare permanet mutations
4 types of DNA repair mechs
mismatch repair
base-excision repair
nucleotide-excision repair
direct repair
cancer
most not inherited, must are mutations that come up from environment
what IS inheritied is poor DNA repair mechanisms
Homologous Recombination
during meiosis
causes inherited sequence diveristy
DNA integration and transposition
provides mech for viruses to invade host genomes
immunoglobin gene rearrangements
somatic cells
generate antibody diversity
Energy and DNA mutation
it takes a lot of nrg to maintain error free genetic info
but its worth it
SEE ENZYMES ON SLIDE 3
SEE ENZYMES ON SLIDE 3
Which three types of enzymes are required in 3 of 4 of the DNA repair mechanisms
DNA liagase
exonuclease
DNA polymerase (1 or 3)
why are these two enzyme types not required for direct repair?
b.c they are direct chem rxns
not removing any nucleotides or bases
Ames Test
to test mutagenic properties of environmental toxins
start with bacteria strain w/ mutation
Ames Salmonella Strain
started with mutation where they can’t live w.o histidine b/c they can’t make it
hes like OH! they need a back mutation
tells them they need to make their own histadine (not provided), most died
then gave them a mutagen, ones closest to disc die, but then they start to adapt to it via mutation
review slide 4
review slide 4
Mismatch Repair
during bacteria replication, parent and daughter strands chem marked. by METHYLATION
new strands not methylated until a few minutes later
IF MUTATION, nascent DNA strand that is unmethylated is cleaved next to the mismatch.
mutated strand removed by exonuclease
replacement done by DNA Pol 3
With mismatch repair, how do we know which one is wrong and what needs to be sliced and replaced?
the unmethylated one! Because it reads this as the new strand
unless it waits to long and then it gets methylated
When Base Excision Repair
happens a lot when cytosine deaminates and becomes uracil (NOT A DNA BASE)
or when Adenine deamination makes inosine
or damaged base
Base Excision Repair
remove only the messed up base
with DNA glycosylase
cleave the glycosyl bond to remove the wrong base
abasic site (site missing base) is recognized by enzyme AP endonclease, this removes rest of nucleotide
DNA Pol 1 and DNA ligase repair the DNA segment
When Nucleotide Excision Repair
due to DNA Lesions (Thymine dimer stacking) that are formed by chemical or UV damage
Nucleotide Excision Repair
uses enzyme ABC exinuclease
cuts on both the 5’ and 3’ sides of them, DNA helicase removes the entire segment of damaged DNA
DNA Pol 1 and DNA ligase repair the gap
When Direct Repair
need to repair methyation of guanine in order to avoid a GC AT mutation
due to DNA lesions again
by UV induced pyrimidine dimers
Direct Repair
directly chemically repairs DNA without removing bases or nucleotides
enzyme is O6methylguanine DNA methyltransferase
Why is colon cancer the first cancer to appear in HNPCC?
Hint: what is the property of intestinal cells (think of nausea from chemo)
Intestinal cells replinish all the damn time
what are most human cancers caused by?
somatic mutations during individual life
SOME are effect of inheritied mutations (most commonly HNPCC, hereditary nonpolyposis colon cancer)
HNPCC due to
mutations in mismatch repair enzymes
autosomal dominant mutation, causes cancer in early age in intestinal tract
Autosomal dominant mutations
due to “gain of function” mutations
interfere with normal protein by forming nonproductive protein complex
so inheritance of just one copy of gene causes disease
you still have one normal protein, but the other one is really bad
DNA recombination provides genetic variation
exchange of DNA sequences between strands (meiosis) or direct integration (pathogen infection)
high rate of recombination during meotic cell division
crossover allows exchange of genetic info between paternal and maternal copies of same chromsomes. similar but not identical
Double Strand Break Model of homologous recombination during meiosis
need to do this so DNA strands can exchange stuff during meiosis
dobule strand break
cuts leave single strand regions
they invade sister chromatid to make new DNA double strand
branch migration determines extent of recombintation between sister chromatids
intertwined DNA molecs cross over at Holiday junctions. MUST BE RESOLVED to avoid tangles
resolution of Holiday Junctions
break and reseal phosphodiester bonds
generates the recombo products
what is a Holiday Junction
the point at which intertwined DNA molecs cross over
must be resolved
what do nucleases do (endo or exo)
chew up DNA
what do ligases do?
seal up DNA
what do polymerases do?
synthesize DNA
