There is a problem I stumbled upon when doing a lesson on molecular genetics (Grade 12 level).

The lesson contains many tables in this format: table

I don't understand how it got to the mRNA codon considering how RNA polymerase creates mRNA. From what I know already RNA polymerase reads the anti-sense/template strand (3`-5`strand) and synthesizes mRNA in a 5`-3` direction.

For example, using the first column with the DNA codon 5`-TAC-3`, I would automatically convert it to 3`-5` complementary anti-sense strand because of the nature of the enzyme. It would be 3`-ATG-5`. Now using that anti-sense strand I would find the complementary base pairs and the mRNA strand would be 5'-UAC-3'. This is not consistent with the example given by the lesson. I can understand how to get to the answer (just find the complimentary base pairs of the DNA codon given) but it goes against what I know.

Now if the table said (3`-5`) instead of (5`-3`) it would make sense. Am I misunderstanding a fundamental concept?

  • $\begingroup$ In addition, I did some research and found this table. en.wikipedia.org/wiki/DNA_codon_table With the example I mentioned, Tyrosine would be the amino acid if I was going directly from the DNA codon not methionine. $\endgroup$ Aug 1, 2018 at 1:30
  • $\begingroup$ The table you reproduced shows the DNA sequence on the bottom or antisense strand of the gene, but the 5’->3’ notation refers to the coordinates on the top or sense strand. $\endgroup$
    – mdperry
    Aug 2, 2018 at 14:52

1 Answer 1


Your reasoning is correct and the table is wrong. The mRNA will have the same base sequence as the sense DNA strand (with T replaced by U).

  • 1
    $\begingroup$ I have upvoted this answer, but would add that — although it is not the poor poster's fault — the term ‘DNA codon’ is non-standard and I can see little point at all in a table of this sort. Perhaps for mRNA codon v. tRNA anticodon, but not for this. $\endgroup$
    – David
    Aug 1, 2018 at 7:50

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