The question
Name: ______________________ Date: ____________ Operon expression with lactose, glucose and tryptophan (9 points)
Data table
Enzyme activity in wild-type and mutant E. coli
The lac operon of E. coli codes for β-galactosidase, which splits lactose; the trp operon codes for enzymes that make tryptophan. Experiment 1: wild-type cells and two mutant strains were grown in media with glycerol (a food that does not affect the lac operon), glycerol plus lactose, or glucose plus lactose, and β-galactosidase activity was measured. Experiment 2: wild-type cells and a mutant strain were grown with or without tryptophan, and the activity of a trp operon enzyme was measured.
| Strain | Glycerol | Glycerol + lactose | Glucose + lactose |
|---|---|---|---|
| Wild type | 3 ± 1 | 980 ± 80 | 55 ± 12 |
| Mutant R (repressor cannot bind the operator) | 950 ± 85 | 990 ± 90 | 60 ± 14 |
| Mutant C (cannot make cAMP) | 2 ± 1 | 70 ± 15 | 52 ± 12 |
| Strain | No tryptophan in medium | Tryptophan added |
|---|---|---|
| Wild type | 400 ± 40 | 15 ± 5 |
| Mutant T (makes no trp repressor) | 410 ± 45 | 395 ± 40 |
(a) (i) Describe what the lac repressor does when no lactose is present.
(ii) Explain how adding lactose leads to transcription of the lac operon. [2 points](b) (i) Identify the independent variable for the wild-type strain in experiment 1.
(ii) Identify the purpose of growing each strain in glycerol alone.
(iii) State the null hypothesis for comparing wild-type β-galactosidase activity in glycerol + lactose with that in glucose + lactose. [3 points](c) (i) Calculate how many times higher wild-type β-galactosidase activity is in glycerol + lactose than in glucose + lactose.
(ii) Using the error bars (± 2 SE), determine whether mutant C's activity in glycerol + lactose differs significantly from wild-type activity in glucose + lactose. [2 points](d) (i) Explain why the wild-type trp enzyme activity falls when tryptophan is added (experiment 2).
(ii) A strain makes a repressor that binds the operator but cannot bind allolactose. Predict its β-galactosidase activity in glycerol + lactose compared with the wild type, and justify your prediction. [2 points]