Lecture 23

Question 1

what happens if the NEW strand slips during replication?

Answer 1

1. newly synthesized strand slips and an extra T base is looped out from the new strand
2. the loop is stabilized by repetitive A sequences on the template strand
3. upon the next round of replication, there will be an extra T-A base pair (INSERTION)

Question 2

what happens if the TEMPLATE strand slips during replication?

Answer 2

1. template strand slips and an extra CT is looped out from the template strand
2. the loop is stabilized by repetitive CT sequences
3. upon the next round of replication, there will be a deletion of C-G and T-A base pair (DELETION)

Question 3

in general, what type of mutation does strand slippage cause?

Answer 3

indels

Question 4

what type of diseases does strand slippage cause?

Answer 4

Trinucleotide Repeat Disorders

Question 5

what happens in Trinucleotide Repeat Disorders?

Answer 5

DNA has CAG repeats that slip and form a loop in the daughter strand, so the repeats get duplicated in the daughter strand

Question 6

what happens if you have more CAG repeats?

Answer 6

greater slippage which creates even more repeats, which causes even more slippage, etc.

Question 7

another name for Trinucleotide Repeat Disorders?

Answer 7

PolyQ disorders

Question 8

why are Trinucleotide Repeat Disorders called PolyQ disorders?

Answer 8

CAG encodes glutamine (Q) so lots of extra glutamine is produced –> leads to abnormal protein folding which leads to protein aggregation and neural degeneration

Question 9

describe trinucleotide repeat expansion in non-coding region of FMR1 gene in Fragile X syndrome (WT vs premutation vs full mutation)

Answer 9

WT: CGG < 45

Premutation: CGG 55-200, so it is more unstable and prone to expansion in offspring but normal phenotype (these are carriers)

Full mutation: CGG >200, so very unstable and lots of methylation which blocks transcription and gives disease phenotype

Question 10

describe trinucleotide repeat expansion in coding region of HTT gene in Huntington’s Disease

Answer 10

severity of disease depends on the number of repeats

Question 11

what does cytosine become when it is deaminated?

Answer 11

uracil

Question 12

describe the consequence of deamination of cytosine

Answer 12

C* is initially paired with G

strand separation + new strand synthesis makes:
- C* paired with A
- C paired with G (WT)

C* paired with A:
- can be fixed to make C-G
- can create T-A pairing which is a permanent error

Question 13

which 2 bases can be depurinated?

Answer 13

1. guanine
2. adenine

Question 14

what happens in depurination?

Answer 14

purine leaves the base –> DNA pol doesn’t know which base to add so it blocks DNA replication and transcription

Question 15

3 ways that mutagens induce mutation

Answer 15

1. replace a base in DNA (base analog)
2. alter a base to induce mispairing
3. damage a base so it can no longer pair

Question 16

describe the effect of intercalating agents on DNA

Answer 16

resembles a base pair so it inserts itself into DNA strand and causes indels

Question 17

describe the effect of UV light on DNA

Answer 17

2 pyrimidines become covalently linked, leading to instability and can block DNA replication

Question 18

describe the two types of ionizing radiation and their effect on DNA

Answer 18

1. X-rays
2. gamma rays

causes ss and ds breaks so it is prone to deletions, duplications, inversions, translocations

Question 19

assay to determine if something is a mutagen

Answer 19

Ames Test

Question 20

how does the Ames Test work?

Answer 20

use cells that have mutation so they cannot grow without histidine

add mutagen –> if mutagenic, will randomly induce mutations that allow the cells to grow in absence of histidine

Question 21

issue with Ames Test and how we can fix

Answer 21

not representative of metabolism, add liver enzymes

Question 22

decribe this graph and the significance

Answer 22

TA100 is His transition mutant
- big increase in colonies so highly affected by Aflatoxin

TA1538, TA1535 are His frameshift mutants
- unaffected by Aflatoxin

therefore, Aflatoxin causes point mutations, not frameshift mutations –> must look at multiple types of mutants to see effect of mutagen