prokaryotic gene expression- lqc 6b

Question 1

The lac operon is an example of gene expression in the bacterium

Answer 1

E.coli

Question 2

The chemical stimulus for switching on the β-galactosidase gene is

Answer 2

lactose

Question 3

Explain what happens when lactose is NOT present in the bacteria’s environment

Answer 3

• A repressor molecule binds to the operator gene
• The enzyme RNA polymerase cannot bind to promotor region
• The β galactosidase gene is switched off , so the gene is not expressed
• Transcription cannot take place, so no mRNA is made
• No β galactosidase is made by translation on ribosomes

Question 4

Explain what happens when lactose IS present in the bacteria’s environment

Answer 4

• Lactose binds to the repressor molecule
• The repressor molecule cannot bind to the operator
  gene
• The enzyme RNA polymerase can bind to promotor region
• Transcription occurs, which makes mRNA
• Translation produces the enzyme β-galactosidase which breaks down lactose into glucose and galactose

Question 5

what experiment can you do that demonstrates prokaryotic gene expression

Answer 5

Uses E.coli bacteria plus a chemical called ONPG

Question 6

why do you use E.coli and ONPG

Answer 6

• Has a similar structure to lactose
• NOT similar enough to bind to the repressor protein
• BUT … similar enough to be broken down by β-galactosidase enzyme into ONP (yellow) and galactose

Question 7

If the lac operon genes are expressed and the β-galactosidase enzyme is made

Answer 7

there will be a colour change from colourless (ONPG) to yellow (ONP)

Question 8

method of experiment

Answer 8

Set up four tubes with different contents. Add 5 drops of methylbenzene to each tube to break open bacterial cells. Shake and leave for 10-20 minutes in a water bath at 35C

Question 9

prediction and explanation of tube containing E.coli bacteria, ONPG

Answer 9

colourless- Repressor molecule binds to operator gene, RNA polymerase can’t bind, gene not transcribed so no mRNA made, no β galactosidase made, ONPG not broken down, no ONP produced

Question 10

prediction and explanation of tube containing

Answer 10

yellow-Lactose binds to repressor molecule, RNA polymerase can bind to operator gene is transcribed/mRNA
made, β-galactosidase made ONPG broken down to ONP (yellow)

Question 11

prediction and explanation of tube containing

Answer 11

colourless- Negative control – for comparison, to prove that ONPG does not break down on its own (without the enzyme)

Question 12

prediction and explanation of tube containing

Answer 12

yellow- Positive control – for comparison, to prove that the enzyme breaks down ONPG