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Name: Matt
Status: Student
Grade:  9-12
Location: CA
Country: United States
Date: July 2005


Question:
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 (staggard configuration).



Replies:
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.

The 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.

vanhoeck


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'

mRNA 5'UUCCAAGG5'

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:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&lis t_uids=12225583&dopt=Abstract

Christopher Perkins



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