Date: September 2004
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.
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
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.
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Update: June 2012