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Name: Ross
Status: Other
Age: 50s
Location: N/A
Country: N/A
Date: September 2004


Question:
I was just reading the answer to a question about the morphology of a gene.

In the answer was this statement: "The promoter is a sequence of nucleotide that the RNA polymerase (an enzyme that builds RNA while reading DNA) binds to start transcription."

How does this process repeat itself? How does the "promoter" or RNA find its way to the start of the gene on the DNA to begin this process? Is it repetious process where, once begun, the RNA created from this process, goes off and does its work and then causes something to happen that starts transcription all over again.



Replies:
The way RNA polymerase molecules "find" the promoter is presumably by random diffusion (thermal motion). I don't know about eucaryotic cells, but in bacteria, RNA polymerase can bind to any promoter that is not being blocked by the corresponding repressor protein molecule. For inducible operons like beta-galactosidase, the repressor is active when there is no lactose present; when lactose is present, it binds to the repressor and inactivates it so transcription can occur. In the case of biosynthetic operons like tryptophan, when tryptophan is present the repressor is activated thus shutting down transcription, and when tryptophan is scarce, the repressor is inactive and transcription can take place. As you suggested there are separate promoters for each gene.

Hope this helps,

Regards, Ron Baker, Ph.D.


There is more than one copy of the RNA polymerase enzyme in a cell. So the same gene can be read over and over by multiple RNA polymerase molecules. The promotor is a sequence of DNA that lies in front of the gene and acts as kind of traffic sign that shows the enzyme where the gene begins. It signals the enzyme that a gene lies ahead on the DNA. The DNA is kind of like a reference cookbook that only resides in the nucleus and can't be "checked out". Let's say you want to borrow Grandma's prize cookie recipe (gene). You are the RNA polymerase that goes to the recipe book and copies it down on an index card (messenger RNA). So you go to the index and look for the page that it is found on (promotor).

Then you take it back to your kitchen to make the cookies (protein). But your cousin also wants the recipe (another RNA polymerase) who then goes to Grandma and makes her own copy. And all of the cousins can do the same. (Multiple RNA polymerases) The original copy of the recipe is still in the cookbook at Grandma's (nucleus). So all of the multiple copies of the recipe (mRNA's) can be making cookies at the same time in various kitchens. Also, multiple batches of cookies can be made from the same copy of the recipe. So the promotor is actually DNA on the chromosome, but just signals the RNA polymerase where to begin copying because it recognizes the sequence. The promotor is fixed on the chromosome and doesn't go anywhere.

vanhoeck



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