Lecture 31: DNA Repair

Question 1

transient mutations

Answer 1

result of:
faulty DNA synth
deamination of cytosine and adenine
environmental damage 
very rare permanet mutations

Question 2

4 types of DNA repair mechs

Answer 2

mismatch repair
base-excision repair
nucleotide-excision repair
direct repair

Question 3

cancer

Answer 3

most not inherited, must are mutations that come up from environment
what IS inheritied is poor DNA repair mechanisms

Question 4

Homologous Recombination

Answer 4

during meiosis

causes inherited sequence diveristy

Question 5

DNA integration and transposition

Answer 5

provides mech for viruses to invade host genomes

Question 6

immunoglobin gene rearrangements

Answer 6

somatic cells

generate antibody diversity

Question 7

Energy and DNA mutation

Answer 7

it takes a lot of nrg to maintain error free genetic info

but its worth it

Question 8

SEE ENZYMES ON SLIDE 3

Answer 8

SEE ENZYMES ON SLIDE 3

Question 9

Which three types of enzymes are required in 3 of 4 of the DNA repair mechanisms

Answer 9

DNA liagase
exonuclease
DNA polymerase (1 or 3)

Question 10

why are these two enzyme types not required for direct repair?

Answer 10

b.c they are direct chem rxns

not removing any nucleotides or bases

Question 11

Ames Test

Answer 11

to test mutagenic properties of environmental toxins

start with bacteria strain w/ mutation

Question 12

Ames Salmonella Strain

Answer 12

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

Question 13

review slide 4

Answer 13

review slide 4

Question 14

Mismatch Repair

Answer 14

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

Question 15

With mismatch repair, how do we know which one is wrong and what needs to be sliced and replaced?

Answer 15

the unmethylated one! Because it reads this as the new strand

unless it waits to long and then it gets methylated

Question 16

When Base Excision Repair

Answer 16

happens a lot when cytosine deaminates and becomes uracil (NOT A DNA BASE)
or when Adenine deamination makes inosine
or damaged base

Question 17

Base Excision Repair

Answer 17

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

Question 18

When Nucleotide Excision Repair

Answer 18

due to DNA Lesions (Thymine dimer stacking) that are formed by chemical or UV damage

Question 19

Nucleotide Excision Repair

Answer 19

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

Question 20

When Direct Repair

Answer 20

need to repair methyation of guanine in order to avoid a GC AT mutation

due to DNA lesions again
by UV induced pyrimidine dimers

Question 21

Direct Repair

Answer 21

directly chemically repairs DNA without removing bases or nucleotides
enzyme is O6methylguanine DNA methyltransferase

Question 22

Why is colon cancer the first cancer to appear in HNPCC?

Hint: what is the property of intestinal cells (think of nausea from chemo)

Answer 22

Intestinal cells replinish all the damn time

Question 23

what are most human cancers caused by?

Answer 23

somatic mutations during individual life

SOME are effect of inheritied mutations (most commonly HNPCC, hereditary nonpolyposis colon cancer)

Question 24

HNPCC due to

Answer 24

mutations in mismatch repair enzymes

autosomal dominant mutation, causes cancer in early age in intestinal tract

Question 25

Autosomal dominant mutations

Answer 25

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

Question 26

DNA recombination provides genetic variation

Answer 26

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

Question 27

Double Strand Break Model of homologous recombination during meiosis

Answer 27

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

Question 28

resolution of Holiday Junctions

Answer 28

break and reseal phosphodiester bonds

generates the recombo products

Question 29

what is a Holiday Junction

Answer 29

the point at which intertwined DNA molecs cross over

must be resolved

Question 30

what do nucleases do (endo or exo)

Answer 30

chew up DNA

Question 31

what do ligases do?

Answer 31

seal up DNA

Question 32

what do polymerases do?

Answer 32

synthesize DNA