|MadSci Network: Genetics|
Hello Thanks for sending in your question. I believe, from what you have asked, that it is merely the terminology used to explain transcription and translation that has confused you. Your question is about what is referred to as the "Central Dogma": i.e. that DNA is read by transcriptional machinery in the nucleus, the information is copied into mRNA, which is transported out of the nucleus, and is then translated into a corresponding protein. i.e. DNA -> mRNA -> protein If there is anyone reading this answer who is unfamiliar with the path from DNA to protein, you might like to refer to the picture at: http://gened. emc.maricopa.edu/bio/bio181/BIOBK/cendogma.gif and some of the related material at: http:/ /gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookGENCTRL.html From your question, the problem you may be having is separating the terminology we use to refer to the information encoded by the DNA, from what happens mechanistically in order to generate a corresponding protein. We often talk about the coding strand, or sense strand of DNA, by which we mean the DNA strand with a sequence of bases that we, as humans, can read along, and, with our knowledge of the relationship between codons and amino acids, predict the amino acid sequence of a peptide. The other strand of DNA can be referred to as non-coding or antisense. Other terminology, which I use below, labels the DNA strands as + (sense) and - (anti-sense) . However, we must remember that in reality there is a nucleotide step between the DNA and protein levels: the mRNA. The translational machinery must be able to read the codons from the mRNA in order to add the correct amino acids to the peptide sequence. So the mRNA must read in the +ve, or sense direction. If we think about how DNA is copied by mRNA, we can see why the terminology used can be misleading: The two strands of DNA are complementary, each base pairing with a complementary base on the opposite strand; each adenine pairs with a thymine, and each guanine with a cytosine. (Uracil is used instead of thymine in RNA.) To copy DNA, or to create mRNA from DNA, the bases of the DNA strand read are paired off with their complementary base. This means that if the+ strand of the DNA was read, a - anti-sense sequence would be produced. However, we need mRNA in the +, or sense direction. To get this, the transcriptional machinery must read the -ve, or antisense DNA strand. And this is where the problem comes in: we, as humans, have named the DNA strands coding and non-coding according to whether we can read the sequence, find the codons, and predict the corresponding peptide sequence. But, from the cellular point of view, the codons encoded by the mRNA have to be readable by the translation machinery, and so must be in the + sense. Because of the way that DNA is transcribed, the transcription machinery must read the - strand of the DNA to produce mRNA in the + sense. So, rather unfortunately, the terminology can cause confusion at first since it is what we have chosen to call the "non-coding" strand of the DNA that is being read by the transcriptional machinery! The trick is to remember that we have named the DNA strands according to what is convenient for us to read. So, to recap: if we assume that in the stretch of DNA we are talking about, only one protein is coded for, then: The + strand is the one we read. The - strand is the one the transcription machinery reads to create a + mRNA strand. The + mRNA strand is read to create the peptide sequence. I hope this answers the question you asked. If not, please do get back in touch.
Try the links in the MadSci Library for more information on Genetics.