MadSci Network: Genetics

Re: does one strand of DNA code for protein and the other strand is redundant ?

Date: Mon Feb 12 15:14:16 2001
Posted By: Bela Tiwari, Staff, Bioinformatics Centre, Oxford University
Area of science: Genetics
ID: 981052593.Ge


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:

and some of the related material at:

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 
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.

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