Mutation - Lecture 3

Question 1

How are the effects of radiation related to the radiation wavelength?

Answer 1

inversely

Question 2

How does UV light cause mutation?

Answer 2

Pyrimidines absorb UV light exciting the molecule which causes bonds to rearrange and pyrimidine dimers to form and DNA poly cannot replicate through these bulky lesions

Question 3

How do X-rays cause mutation?

Answer 3

Ionization causes chromosome breaks and changes in chromosome structure

Question 4

What are the 3 types of Direct reversal of DNA damage?

Answer 4

Light-dependent repair, enzymatic removal of alkyl, and ligation of DNA nicks

Question 5

Describe light-dependent repair.

Answer 5

only in prokaryotes, uses photolyase that binds to pyrimidine dimers and breaks the bond to reform the correct base pairing

Question 6

Describe enzymatic removal of alkyl.

Answer 6

in both prokaryotes and eukaryotes, reverses alkylation (removes methyl group), reestablishing correct base pairing

Question 7

Describe ligation of nicks in DNA.

Answer 7

in both prokaryotes and eukaryotes, removes single-stranded nicks

Question 8

What is base excision repair?

Answer 8

found in both prokaryotes and eukaryotes, removed DNA bases damaged by deamination, alkylation, or oxidation

Question 9

Describe the pathways of base excision repair.

Answer 9

DNA glycosylase removes a damaged base producing an AP site
AP endonuclease cleaves the phosphodiester bond on the 5’ side of the AP site and removes the deoxyribose sugar
DNA polymerase adds new nucleotides to the exposed 3-OH’ group
The nick in the sugar-phosphate backbone is sealed by DNA ligase restoring the original sequence

Question 10

What is nucleotide excision repair?

Answer 10

found in both prokaryotes and eukaryotes, removes PD and other bulky DNA damage that blocks replication

Question 11

Describe the pathways of nucleotide excision repair.

Answer 11

An enzyme complex recognizes the distortion resulting from damage
The DNA is then separated and SSB proteins stabilize the single strand
An enzyme cleaves the strand on both sides of the damage
The gap is filled by DNA poly and sealed by DNA ligase

Question 12

What are the similarities between base and nucleotide excision repair?

Answer 12

DNA repair endonuclease binds and excises the damaged base
DNA polymerase fills in the gap
DNA ligase seals the break

Question 13

What is mismatch repair?

Answer 13

removes mismatched base pairs developed as a result of a DNA poly error

Question 14

Describe the pathway of mismatch repair.

Answer 14

DNA is scanned for the semi-methylated GATC sequence, only the parent has this sequence so they new strand is identified as the one without, the new strand is digested by an exonuclease to remove incorrect nucleotide, DNA poly and lagase fill and seal the nick

Question 15

Descrive homologous post-replication recombination repair.

Answer 15

Newly-synthesized portion of the leading strand unwinds from the top template strand and pairs with the newly-replicated section of the lagging strand as the template can now proceed past the point where the PD is located
The PD is then later removed by another DNA mechanism

Question 16

Describe translesion synthesis DNA polymerase.

Answer 16

TLS DNA poly can be substituted for the blocked main replicative polymerase
The TLS DNA poly can replicate through bulky DNA
After the main polymerase can come back into the process on continue replication
In both prokaryotes and eukaryotes

Question 17

What is the down-side of TLS DNA polymerase?

Answer 17

it is extremely error prone and therefore only used an an SOS response