Modeling the Operon

Class began with a warmup of the trp operon.  You were asked to explain what happens when trp is present in the cell using the terms: regulatory gene, structural genes, RNA polymerase, operator, repressor, promoter and transcription.

We then did a brief comparison of the lac operon to the trp operon.  The important differences are:

• the lac operon is INDUCIBLE (usually off, but is turned on by lactose) while the trp operon is REPRESSIBLE (usually on, but is turned off by tryptophan)
• the lac operon is for the breakdown of lactose, while the trp operon is for the construction of tryptophan.
• in the lac operon the repressor is constructed in an ACTIVE form (which is why the operon is usually off) while in the trp operon the repressor is constructed in an INACTIVE form (which is why the operon is usually on)

Students then made simple models of the lac operon that could be manipulated to show what happens when lactose is present or absent in the bacterium.  These models will be used on block day to demonstrate comprehension.

There is no assigned homework.  Our next topic is to expand our study of control of gene expression to eukaryotes.  We will also be talking about the positive control of the lac operon that also exists.

For class tomorrow, be prepared to answer these questions:

1. What is the function of the operator?
2. What is the function of the promoter?
3. What is the function of the repressor?
4. In the lac operon, is the repressor active or inactive when lactose is present?
5. In the trp operon, is the repressor active or inactive when tryptophan is present?
6. What does it mean when an operon is said to be "repressible?"
7. What is the product of the trp operon?
8. What is the product of the lac operon?
9. Name the specific organism we use to study the lac and trp operons.