RNA Transcript and DNA
Country: United States
Date: July 2005
A DNA strand splits and one side is templated with RNA
containing the code to create a protein.
The other side of the DNA strand is complementary so does not code for
this protein. So my question is...are all RNA transcriptions done on only
one of the DNA strands or are the RNA's transcribed on each strand
RNA reads only one side of the DNA but the code can be contained in the
reverse direction on the "opposite" strand. Computer algorithms have been
written to scan the DNA in the Human Genome Data banks and the code can be
found in any of 6 different reading frames. Recall that a codon is three
letters of RNA. But when you look at a sequence of DNA you don't know where
that sequence has begun. It could have begun with the first letter of a
codon, but it could also be starting with the second or third. Also, RNA
must read the DNA in the 5' to 3' direction, which goes from left to right
on the "top" strand, but recall that DNA strands are antiparallel. So the
other side of the DNA has the code running in the opposite direction.
same thing can happen on the other strand so there are also 3 reading frames
on that side. What computers look for are start codons (ie. AUG which codes
for methionine) and stop codons and long strings of amino acids in between.
There are also other clues computers are "taught" to look for, so it's of
course more complicated, but that's the simple answer. Also, eukaryotic DNA
doesn't contain overlapping code (maybe one or two letters), so there
aren't' staggered genes.
Only one of the strands is transcribed into mRNA and it is called
paradoxically the antisense strand. The other strand, called the sense
strand, serves only as a template for the synthesis of the antisense strand.
The so-called sense strand is given that designation because when the gene
is transcrbed into mRNA, it ends up with the same sequence as the sense
strand except that U's replace T's. If the sense strand was transcribed into
mRNA, it would code for an entirely diferent protein than the protein it was
designed to code for. For example, if the antisense strand sequence was
5'AAGGTTCC3', the resulting RNA would have the sequence 5'GGAACCUU3'. If the
sense strand was transcribed, it would result in the sequence 5'AAGGUUCC3'.
Remember, the convention for writing DNA and RNA nucleotide sequences is
from the 5' end to the 3' end and that all nucleic acid sequences are
synthesized in the 5' to 3' direction starting at the 3' end of the template
strand. You must also understand that the two complementary strands of DNA
are anti-parallel meaning they are parallel but run in opposite directions.
sense strand 5'GGAACCTT3'
antisense strand 3'CCTTGGAA5'
Ron Baker, Ph.D.
After reading the above replies, I think some of my colleagues have
misunderstood your question. I understand your question to ask whether
all mRNA transcription - i.e. transcription of different genes - takes
place on the same strand.
While my colleagues are correct to point out that only one strand is
transcribed within a particular gene, in fact the next gene down the
line may be transcribed from the other, complementary, strand. If we
call one strand "Watson" and the other "Crick" (because molecular
biologists are notoriously fond of silly inside jokes), we can say that
some genes reside on Watson and others on Crick. Of course, because the
strands are antiparallel (running in opposite directions), RNA
polymerase enzymes will travel in one direction on Watson and the other
direction on Crick.
Furthermore, there is a lot of fairly recent evidence that, in fact,
there are a few instances of overlapping but oppositely-oriented
transcribed sequences. These have been found in both human and mouse
genomes, and it is reasonable to suppose that they occur in all
eukaryotes. See the following article:
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Update: June 2012