Bacteria

Question 1

Suggest the advantages of regulating the lac operon [2]

Answer 1

1. Economic use of energy and resources
   » genes expressed only when necessary
2. Respond rapidly to environmental changes

Question 2

Explain where mutations could have occurred for
A: constitutive expression even in presence of Arg
B: non-expression of gene A only
C: no expression even in absence of Arg

Answer 2

A: operator / repressor gene / repressor promoter

B: Gene A
» mRNA is polycistronic
» each gene has its own start and stop codon
» Tsl occurs independently

C: Promoter

> > REJECT: repressor genes (super-repressor)

• valid for lac repressor (active by default)&raquo_space; unable to bind allolactose
• but LOF mutation (for Arg) is unlikely

Question 3

Draw and label the lac operon and its associated regulatory gene [3]

Answer 3

Regulatory gene (P + lacI)
---
Promoter
Operator
Structural genes (lacZ/Y/A)

Question 4

Compare polycistronic mRNA with eukaryotic pre-mRNA

Answer 4

1. Start and stop codons - several vs. one

2. Introns

Question 5

Describe conjugation [3]

Answer 5

1. SEX PILUS of F+ makes contact with F- cell
   » retracts&raquo_space; MATING BRIDGE formed
2. One strand of F plasmid is nicked&raquo_space; transferred
   » the un-nicked strand used as a template for elongation by DNA POL
3. ss F plasmid DNA circularises in F- cell
   » used as TEMPLATE to synthesise complementary strand by DNA POL&raquo_space; ds F plasmid
   » F+ cell with new alleles

Question 6

Describe specialised transduction [3]

Answer 6

1. TEMPERATE PHAGE infects&raquo_space; integrates during LYSOGENIC CYCLE
2. Induction&raquo_space; lytic cycle&raquo_space; prophage excised with adjacent bacterial genes
3. Infection&raquo_space; homologous recombination