10.3 Mutations Ames Test

Question 1

Classifications of point mutations

Answer 1

By effect on amino acid sequence:
- Silent (synonymous)
- Missense (nonsynonymous)
- Nonsense

Question 2

Classifying nucleotide insertions/deletions (indels)

Answer 2

Inside reading frames: cause frame shifts that have either nonsense or missense effects (except when they occur as multiples of 3 nucleotides).
Outside reading frames: no effect on phenotype.

Question 3

Classifying mutations by effect of functional phenotype

Answer 3

Loss of function: recessive inheritance.
Gain of function: new gene product, dominant inheritance.
Neutral: missense mutation that results in non-significant changes in protein function.

Question 4

Transition point mutations

Answer 4

Change in purine to a purine, or a pyrimidine to a pyrimidine.
Eg., A to G, C to T…

Question 5

Transversion mutations

Answer 5

Change in nucleotide from a purine to a pyrimidine or vice versa.
Eg., A to C, G to T…
Less common that transitions even though there are twice as many combinations.

Question 6

Forward mutations

Answer 6

Alters the wild phenotype

Question 7

Reverse mutations

Answer 7

Changes mutant back to wild phenotype.

Question 8

Suppressor mutations

Answer 8

Where the first mutation is suppressed by a second mutation, intragenic or intergenic.

Question 9

Three types of spontaneous mutations

Answer 9

• Tautomeric shifts
• DNA strand-slippage during DNA replication
• Misalignment of homologous chromosomes during MI

Question 10

Tautomeric shifts

Answer 10

Standard base pairing is switched to base pairing with rare forms of a nucleotide causing anomalous pairing (C to A, T to G, etc). Result in incorrect base-pairing during DNA replication.

Question 11

DNA strand-slippage

Answer 11

An insertion or deletion due to mispairing. Newly synthesised strand slips out and creates an addition OR the template strand loops out resulting in an omission.

Question 12

Misalignment during crossing over of meiosis I

Answer 12

One crossover product contains an insertion and the other has a deletion.

Question 13

Agents that cause mutation

Answer 13

Radiation
- ionising: X-rays, cosmic rays, gamma rays.
- ultraviolet radiation
Chemical
- base analogs
base modifying agents
intercalating agents

Question 14

Ionising radiation

Answer 14

Change stable molecule into a free radical or an ion by dislodging an electron; can alter the structure of bases and break the phophodiester bonds.

Question 15

Ultraviolet radiation

Answer 15

Electromagnetic radiation of lower energy but that can still generate free radicals under some circumstances.

Question 16

Pyrimidine dimers

Answer 16

Thymine dimers induced by ultraviolet radiation, can block DNA replication by causing a kink in the backbone.

Question 17

Xeroderma pigmentosum

Answer 17

An autosomal recessive genetic disorder in which the ability to repair mutations caused by UV light is deficient.

Question 18

5-Bromouracil

Answer 18

A nucleotide analog that resembles both thymine and cytosine. Can base pair with adenine but will also pair with guanine when ionised.

Question 19

Base modifying mutagens

Answer 19

Modify the groups on the normal bases in DNA that results in incorrect pairing to introduce mutations during replication

Question 20

Intercalating agents

Answer 20

Planar chemicals that distort the normal stacking by 0.68nm (the size of a base), causes insertions/deletions during DNA replication.

Question 21

The Ames test

Answer 21

Method to measure the reversion of a mutant His- Salmonella strain to the wild type by potential mutagens.
His- cannot grow on minimal medium lacking histidine.
His+ will grow on minimal medium.
Increased reversions indicate the chemical is a mutagen, and thus, a potential carcinogen.

Question 22

Ames test rat liver enzymes

Answer 22

Enzymes mimic the chemical modification of potential mutagens in the human body. Could make the chemical more or less mutagenic.