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Discus: SAT/ACT Tests and Test Preparation: May 2004 Archive: AP BIO HELP PLEASE FELLOW CCers
By Reject (Reject) on Sunday, May 09, 2004 - 09:31 am: Edit

i really need help. I have trouble understanding gene control and negative and positive control of transcription with all that promoter, operator, stuff...

can anyone explain that process in a simple way.


By Jenesaispas (Jenesaispas) on Sunday, May 09, 2004 - 09:53 am: Edit

I can try.

There are two types of operons. I'll explain the two most well-known ones in Escherichia coli.

Operons consist of four things: a regulator gene that produces a repressor; a promoter to which RNA polymerase attaches for transcription; the operator, to whcih the repressor protein binds; and the structural genes, which are the "normal" genes that code for peptides, etc.

Okay. The lac operon breaks down a form of lactose (disaccharide) into usable energy (glucose and galactose, which are monosaccharides). When lactose isn't present in the cell, there is no need for the genes to be "turned on." So, normally, the repressor protein binds to the operator and prevents RNA polyeramse from attaching to the promoter. Thus, the genes are not expressed.

However, when there is lots of lactose around E. coli, it needs to break down the lactose. So, some of the lactose binds to the repressor (kind of like an allosteric site in an enzyme) and the repressor changes shape. Now, it is unable to bind to the operator and RNA polymerase CAN attach to the promoter, so transcription occurs and the enzymes are synthesized to break down lactose. E. coli can now surive! (This is an inducible operon because lactose must "turn it on.")

The trp operon is essentially the opposite. The trp operon controls the synthesis of tryptophan, an amino acid. Usually, E. coli must make its own tryptophan. This means that the repressor is inactive and is unable to bind to the operator, which means RNA polymerase can produce the enzymes to make tryptophan. When tryptophan happens to be present in the cell, some of it binds to and activates the repressor. The repressor changes shape and is able to bind to the operator. Thus, RNA polymerase cannot attach to the promoter and transcription will not occur--which is good, as there is already tryptophan in the cell. This ia a repressible operon, since the substance causes it to "turn off."

I hope that helped.

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