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I am having problems getting the ‘correct’ answer for the above question.

I am assuming that mRNA uses the template strand (not the coding strand) for protein synthesis. So:

         ---------[DNA]  -  [mRNA]  - [Amino Acid]                       

Original A T A -> U A U -> tyrosine
Mutated T C T -> A G A -> Arginine

What is my mistake?

  • 2
    $\begingroup$ Welcome to the site. Please visit the help center and take a tour. One of the requirements for questions on this site is that they demonstrate that they have done the research to look for the answer themselves. Think about what the question is asking and what the word coding means... $\endgroup$
    – bob1
    Apr 22, 2022 at 3:09
  • 1
    $\begingroup$ Look up "coding strand" in wikipedia. $\endgroup$
    – swbarnes2
    Apr 22, 2022 at 16:21

1 Answer 1


The problem here is the misleading nature of the term “coding strand”.

I have previously posted on this topic making the point that mRNA — which is what the genetic code relates to — should be the basis of nomenclature, and that the best and least ambiguous terminology is sense (the way the code is read from the mRNA) and antisense (its reverse complement). To quote myself:

The idea seems to me clear that when you read the string of codons that encode the amino acid sequence from this strand they ‘make sense’. (Anti-sense is used in preference to non-sense, as ‘nonsense’ was the term used historically for mutations that converted amino acid codons into stop codons.) It can also be extended to non-protein coding genes (e.g. for tRNA), where ‘sense’ correlates with the sequence of the gene product.

Thus, if you consider the two strands of DNA, the one that is transcribed by RNA polymerase into DNA is the anti-sense strand, and the other (which is the one humans may read after a DNA sequencing experiment) is the sense strand.

My main objection to the use of coding and non-coding for DNA strands (apart from logic — how does one code and the other not?) is ambiguity:

As the anti-sense strand is the template for RNA polymerase, one might make the mental association between this and ‘coding’, whereas it is the sense strand that is meant [by coding] in this (admittedly common) usage.

So in my opinion the question is to blame, although this is very common. But, to answer the question, one presumably should assume that the strand with the asterisk is the sense strand, and any changes in it are reflected directly (apart from T to U) in the mRNA.


It is ironic that the genetic code provided in this question is unlikely to be correct, although the organism is not specified. For example, in vertebrates the following applies:

 AGA    Stop (not Arg)
 AGG    Stop (not Arg)
 AUA    Met  (not Ile)
 UGA    Trp  (not Stop)



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