One of my study questions for a biology test was:

A gene contains 1200 bases. How many codons will be on the mRNA immediately after transcription?

My initial answer was 400 codons, because 1200 bases = 1200 nucleotides and 1200/3 = 400. But since DNA is double helix, and only one of the two strands is used in mRNA creation, would only 600 bases be used instead of the full 1200?

  • $\begingroup$ My instinct would be to divide the number of bases by 3 like you did. I've never seen gene length described as twice the number of base pairs. $\endgroup$
    – user137
    Commented Feb 22, 2016 at 3:06
  • $\begingroup$ @user137 Do you mean that the number of bases in a gene are usually counted using one strand (in double helix DNA)? $\endgroup$
    – user22047
    Commented Feb 22, 2016 at 3:08
  • $\begingroup$ Yes, if a gene has 1000 base pairs, we refer to the sense strand, the other strand is implied. Of course we typically use the term "base pairs", and maybe your professor is being pedantic about bases vs base pairs. You won't know for sure until you get the test back. $\endgroup$
    – user137
    Commented Feb 22, 2016 at 4:19
  • $\begingroup$ I believe the question should have been "A gene contains 1200 base pairs. ...." $\endgroup$ Commented Oct 23, 2016 at 5:29

1 Answer 1


Assuming that all 1200 bases in the gene code for mRNA, then 1200/3 = 400 is the correct calculation.

It's important to understand that a gene will only ever be transcribed from one strand of double-stranded DNA. Think about it mechanistically - first, DNA is transcribed to RNA by a polymerase that can only move in one direction. Second, the strand complementary to a gene's transcription start site is that sequence's complement and therefore not a start site itself. Hope that clears it up. :)


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