Discussion Assignment #9

To Transcribe or Not to Transcribe

One of the great achievements of the last half century has been the cracking of the gene control mechanism in prokaryotic cells. Eukaryotic cells are a lot more complicated, but understanding how this is done in bacteria and viruses has helped to crack open the black box of eukaryotic gene control.

First, a bit of concept investigation. What is an operon? What important parts are usually included in an operon? What is the difference between a constitutive and an inducible system?

Jacob and Monod, who discovered operons, studied the lac operon in E. coli. Normally, the sugar utilized for production of metabolic energy is glucose, but if glucose is scarce, other sugars can be utilized. One of these is lactose. Why would it be desireable for the genes which produce enzymes necessary for the metabolism of lactose to be indicible, rather than constitutive? Under what conditions do you want these genes to be "turned on" (assuming you're a hungry E. coli).

Diagram and explain the normal function of the lac operon. Do *not* copy and paste from another document--I want your diagram, not somebody else's.

Okay, this one requires that you put your thinking and reasoning skills to work. Bacteria usually contain only one copy of each of their genes (as they usually have only one DNA molecule). However, it is possible to make a bacterium diploid for part of its genome by getting the cell to pick up a plasmid carrying these genes. (A plasmid is a small circlet of DNA--bacteria typically carry an assortment of these, and they are frequently utilized for genetically engineering bacteria.) A number of experiments were done with bacteria which had been made diploid for all of the components of the lac operon.

Assume that we've got two strains of these altered bacteria (they contain two copies of the lac operon). Read the following descriptions carefully, and think about the way this system works before you attempt to answer my questions.

We have two of these altered strains. In both of our strains, the following is true:

In one of their lac operons (which we'll call the first lac operon), gene lacZ (the beta-galactosidase gene) is completely normal, but gene lacY (the permease gene) contains a mutation which prevents the gene product from functioning.

In their second lac operons, the lacZ gene is mutated, an dproduces no functional gene product, but the lacY gene is perfectly fine. Like this:

Okay, here's the tricky part. Remember that both strains contain both of the lac operons described above in each cell.

In Strain Number One, the lacI gene (which produces the repressor) on the DNA carrying the first lac operon has mutated and is unable to produce functional repressor. The lacI gene which is on the DNA carrying the second lac operon is normal and produces perfectly functional repressor. Everything else works correctly. Like this:

Strain Number One

In Strain Number Two, both of the repressor genes function normally (produce correctly functioning repressor proteins). But in this strain, the operator on the second lac operon is damaged and is unable to bind to any repressor molecules. Everything else works correctly. Like this:

Strain Number Two

All right, here, finally are your questions. Compare expected performance for these two strains of bacteria. In particular, tell me (a) whether they are able to produce both gene product Z and gene product Y, and (b) whether the production of each of these two enzymes will be constitutive or inducible.

Have fun LOL!

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