Lecture 2

Question 1

Finish the sentence

Resistance is …

Answer 1

An inheritable genetic trait

Question 2

Explain the induced mutation idea for mutant bacteria

Answer 2

That the presence of phage induced some bacteria to become resistant. This is an adaptive response

Question 3

Explain the spontaneous mutation idea for mutant bacteria

Answer 3

Resistance mutations arose spontaneously and were selected for following bacteriophage addition

Question 4

What did the Luria and Delbruck experiment look at and see?

Answer 4

Looked at TI bacteriophage that infects/kills E.Coli and saw most E.Coli died but few were resistant and survived. These bacteria gave rise to more resistant offspring

Question 5

If a phage induced the mutation what would the fluctuation test show?

Answer 5

That there should be a similar number of mutations across the cultures when the phage is introduced as they have all been exposed to the same environmental pressure.

Lower fluctuation

Question 6

If spontaneous mutation occurred, what would the fluctuation test show?

Answer 6

Across the cultures, resistance to phage will arise prior to introduction of the phage.

If mutation occurs earlier = higher number of colonies due to more being resistant,

If mutation occurs in later generations = fewer have resistance

Higher fluctuations

Question 7

What did the fluctuation test conclude?

Answer 7

That mutations arise spontaneously and randomly and can be selected for environmentally

Question 8

What are the limitations to the experiment?

Answer 8

1. The bacteria are killed rapidly therefore have no time to adapt
2. Can take several generations for resistance mutation phenotype to emerge in population

Question 9

What is the most direct way to test for mutagens?

Answer 9

Lab animals as they are a good genetic model for humans

Question 10

What are the issues with testing on animals?

Answer 10

• Costly
• Unpractical as would require to screen large numbers
• Sensitivity - how would you detect the mutations?
• Unethical

Question 11

What is the alternative approach to testing mutagens?

Answer 11

Using micro-organisms

Question 12

What are the benefits to using micro-organisms?

Answer 12

• Genes work in same way as eukaryotes
• Can screen large numbers at once
• Sensitive screens available for detecting mutation occurrence
• Mutation pathway well understood
• No ethical issues

Question 13

Many chemicals are not mutagenic to bacteria but are to animals. Why is this and how did they solve this?

Answer 13

1. The liver in animals converts the chemicals to active forms which are mutagenic post ingestion
2. Prepare liver extracts and incorporate them into the medium used in the Ames test

Question 14

What are the 3 added features of bacterial test strains?

Answer 14

1. Mutation in gene (rfa) affecting CELL ENVELOPE making bacteria MORE PERMEABLE
2. Defective gene (uvrB) encoding protein that REPAIRS DAMAGED DNA and DECREASES FREQUENCY OF MUTATIONS
3. Contain a plasmid (pkM101) that INCREASES EFFECTIVENESS OF SOME MUTAGENS

These three features increase effectiveness of mutagens in causing mutations