Spontaneous and Induced Mutations

Question 1

What was the aim of Luria and Delburk’s experiment?

Answer 1

To see if mutations were spontaneous or a result of environmental pressure

Question 2

What should we expect if the stimulus triggers a mutation in the DNA of the bacteria?

Answer 2

Depending on the time at which we cause the stimulus/ phage we should expect a fixed number of bacteria that will be resistant

Question 3

What did Luria and Delburk’s experiment demonstrate?

Answer 3

Mutations are not induced by the environmental stimulus but rather by occuring randomly in the population of the bacteria

Question 4

What is the spontaneous mutation rate in eukaryotes?

Answer 4

10^-4–> 10^-6 per gene per generation

Question 5

What happens to most spontaneous errors?

Answer 5

They are corrected by proof-reading activity of DNA polymerase

Question 6

What can DNA replication errors be?

Answer 6

1. Point mutations: small insertions or deletions
2. Base-pair substitution mutations can result from “wobble” pairing

Question 7

What is “wobble” pairing?

Answer 7

Pairing within bases which is not the standard Watson-Crick base pairing

Question 8

Give examples of “wobble” pairing

Answer 8

1. Base pairing between normal pyrimidines and rare forms of purines [A G]
2. Base pairing between rare forms of pyrimidines [C T] and normal forms of purines

Question 9

What do we get in the 2nd generation progeny as a result of mismatch caused by wobble base pairing?

Answer 9

Production of one mutant and 3 wt

Question 10

When is it deletion?

Answer 10

If it is on the template strand

Question 11

When is it insertion?

Answer 11

If it occurs on the newly-synthesised strand

Question 12

Give examples fo spontaneous chemical changes

Answer 12

Depurination
Deamination

Question 13

Depurination

Answer 13

Removal of the purine from DNA by breaking the bond with its deoxyribose in the backbone

Question 14

Deamination

Answer 14

Removal of the amino group from a base [cytoine to uracil]

Question 15

How can depurination and deamination be repaired?

Answer 15

Base excision repair system

Question 16

Induced Mutations

Answer 16

Mutations induced in our DNA because of exposure to chemicals called mutagens

Question 17

How can we induce mutations?

Answer 17

1. Radiation
2. Ultraviolet: strongly absorbed by purines and pyrimidines forming abnormal chemical bonds called thymine dimers

Question 18

Classes of chemical mutagens

Answer 18

1. Base analogs
2. Base modifying agents
3. Intercalating agents

Question 19

Base analogs

Answer 19

Incorporates a base in an alternative state in front of a base analog, which will lead to a base change

Question 20

Describe 5-bromouracil as an example fo a base analog

Answer 20

5BU has a bromine residue instead of a methyl group of thymine so it resembles thymine and incorporates itself in DNA during replication.

In its rare state 5BU pairs only with guanine [3 hydrogen bonds] resulting in a TA-> CG transition mutation

Question 21

Base modifying agents

Answer 21

Agents that can induce mutations at any stage of the cell cycle

Question 22

3 types of base modifying agents

Answer 22

1. Deaminating agents remove amino groups [nitrous acid]
2. Hydroxylating agents
3. Alkylating agents that add alkyl groups to bases

Question 23

Intercalating agents

Answer 23

Used in the lab to label DNA/ RNA during electrophoresis as they intercalate between bases

Question 24

Discuss the process used by intercalating discs

Answer 24

1. At replication, a template that contains an intercalated agent will cause invertion of a random extra base
2. The base-pair addition is complete after another round of replication during which the intercalating disc is lost
3. If an intercalating agent inserts into new DNA in place of a normal base, the next round of replication will result in a deletion mutation
4. Result in frameshift mutations

Question 25

What is the Ames test?

Answer 25

Screens for potential carcinogens

Question 26

How is the Ames test carried out?

Answer 26

We start from a strain that is histidine -. We take the strain and mix it with liver extract from a rat and place the bacteria into the chemical and we create 2 plates: non-permissive and permissive one

in the permissive plate, we will have normal growth of the bacteria while in the non-permissive we won’t have bacterial growth

We might have a situation where the bacteria grows in teh medium where it wasn’t supposed to–> MUTATION

Question 27

Nutritional mutations

Answer 27

Auxotrophic mutants are easily detectable for microorganisms that normally can grow on minimal medium

Question 28

Replica plating

Answer 28

Cells are first grown on supplemented medium and then the colonies are transferred to minimal medium as well as to control a plate of supplemented medium. Colonies that grow on the supplemented but not on media are selected for further study

Question 29

Conditional mutations

Answer 29

Mutant that you observe the phenotype when they are in a specific condition i.e. heat sensitivity