Topic 3: Gene expression in prokaryotes (bacteria)

Question 1

What are the steps for gene expression?

4 steps

Answer 1

1. RNA polymerase and {regulator protein/transcription factor} bind – gene switched on
2. Gene can be expressed
3. Transcription produces mRNA
4. Translation produces protein

Question 2

What are the lac operon? (1 point)
What do they make? (1 point)

What do E.coli use this product for? (1 point)

Answer 2

1. cluster of genes in E.coli,
2. which are transcribed and translated to produce the enzyme β-galactosidase
3. used to break down/digest lactose

Question 3

Key Summary: How does E.coli ensure that no enzyme is made to break down lactose, when lactose is not present?
Explain how the lac operon gene expression is regulated.

(6 steps)

Answer 3

1. repressor molecule binds to the operator gene in the promoter region
2. RNA polymerase enzyme cannot bind to promoter region
3. the β galactosidase gene is SWITCHED OFF

4, no mRNA is made by transcription

5. no β galactosidase enzyme (protein) is made by translation on ribosomes
6. bacteria save energy and amino acids

Question 4

Key Summary: How does E.coli ensure that an enzyme is made to break down lactose when lactose is present?

(6 steps)

Answer 4

1. Lactose binds to the repressor molecule
2. This stops the repressor binding to the operator gene in the promoter region
3. the β galactosidase gene is SWITCHED ON
4. mRNA is made by transcription
5. β galactosidase enzyme is made by translation on ribosomes
6. enzyme breaks down lactose into glucose (used for respiration to release energy) and galactose

Question 5

List the 3 properties of ONPG that are useful for the lac operon experiment

Answer 5

1. Has a similar structure to lactose
2. not similar enough to bind to the repressor protein
3. similar enough to be broken down by b-galactosidase enzyme into ONP (yellow) and galactose

Question 6

Describe the method for the lac operon demo

set up 4 things
+
(3 steps)

Answer 6

Set up four tubes

Tube A: 1 cm3 E.coli bacteria only

Tube B: 1 cm3 E.coli bacteria + lactose

Tube C: 1 cm3 distilled water

Tube D: 1 cm3 β-galactosidase enzyme solution

Add to each of the 4 tubes:

1. 5 drops of methylbenzene – breaks open bacterial cells
2. 1 cm3 of ONPG
3. Shake and leave for 10-20 minutes in a water bath at 35C

Question 7

What should be the result of the lac operon demo

2 points

Answer 7

1. In any tube that contains the β-galactosidase enzyme at its optimum temperature, there will be a colour change from colourless to yellow
2. So, if the lac operon genes are expressed and the β-galactosidase enzyme is made, there will be a colour change from colourless (ONPG) to yellow (ONP)