On Wed, 20 Oct 2004 10:59:33 -0700, "Brandon" <chefaudet at yahoo.com>
wrote:
>>>>> The RNA polymerase, the enzyme that makes the RNA copy, recognizes
>> start sites, which are on the correct strand. Look up "promoter".
>>Right, I have read about the promoter, but I am hung up on the part where
>there is only one codon for the start code. So how is it that the
>polymerase gets the correct strand given that the same codon is in multiple
>locations? Does it already "know" what the base sequence is going to look
>like and there fore skips over obviously incorrect base sequences, or are
>the codons different on DNA than they are on RNA, resulting in more variety
>of start and stop codons than just start= AUG, Stop = UAA,UAG,UGA as in RNA?
>
Brandon,
The promoter and the start codon are unrelated. The former is the
recognition site for starting RNA synthesis; the latter is the start
site for protein synthesis (and is basically irrelevant to RNA
synthesis, thus is irrelevant to what you are asking).
The promoter is long enough to adequately deal with the problem. That
is, you have properly described the problem. Not surprisingly, biology
has figured it out. The promoter is asymmetric, and is long
enough/rare enough that (most) transcription only starts in the right
places.
Once an mRNA has been made, a simple model is that translation then
starts at the first start codon. Actually, in the real world it is
more complex than that, but that is a simple idea to start with.
Do you have any molecular biology or microbiology book?
bob
