A bacterium has no glucose. Say, for each case, whether the repressor is on the operator, whether…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A bacterium has no glucose. Say, for each case, whether the repressor is on the operator, whether the operon is on, and whether the enzymes are made.
- There is no lactose in the medium.
- Lactose is added to the medium.
- Why is a very small amount of permease present in the cell even before lactose is added?
- Why is glucose not an inducer of the operon?
Need a reminder? The key ideaShow
In the lac operon, lactose binds the repressor and takes it off the operator, and then the operon is on. In a DNA fingerprint, half of the bands of a child come from its mother. The other half come from its father. So every band of the child must be found in one of the two parents.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The repressor, made by the i gene, is on the operator. The operon is off, and almost no enzyme is made.
Step 2. (b) Lactose binds the repressor, and the repressor leaves the operator. The operon is on. Beta-galactosidase, permease and transacetylase are made.
Step 3. (c) The operon leaks a little all the time. This small amount of permease lets the first lactose enter the cell.
Step 4. (d) The inducer must fit the repressor and take it off the operator. Lactose does this, and glucose and galactose do not.
Answer: (a) off; (b) on; (c) a very low level is always made; (d) glucose does not bind the repressor.
Check. This is a negative regulation, because the repressor switches the operon off.
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