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I am a Biology novice so I apologize in advance if I state anything incorrectly....

My question is, are the DNA codon mappings arbitrary? I understand that the DNA code is made up of codons which each consist of 3 base pairs. There can be 64 codons which map to 20 amino acids (some are duplicated and others act as stop codons). So was there a reason why one particular codon mapped to one particular amino acid - or is the mapping completely arbitrary?

Thanks!

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  • $\begingroup$ "the assignment of codons to amino acids seem to be an advantageous one (most likely not the optimal one)" - so why didnt the optimal one appear at random ? $\endgroup$
    – Pilgim
    Commented Jun 21, 2018 at 13:00
  • $\begingroup$ The code is for RNA, not DNA. As a novice you need to understand this basic point. $\endgroup$
    – David
    Commented Jun 21, 2018 at 17:42

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There is no reason for why a certain codon came to represent a certain amino acid. But some reason is attributed to similarity of degenerate codons. Usually the degenerate codons only differ in their last (third) nucleotide. The corresponding first anticodon in tRNA usually bears a modified nucleotide such as inosine which can (promiscuously) pair with A, C and U.

You can see the wikipedia page on wobble base-pairing.

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Evidence suggests that the correspondence between DNA codons and amino acids (i.e. the genetic code ) is not random. A good place to start with this interesting topic is the Wikipedia article about the genetic code, in particular, about its origin:

Many hypotheses on the evolutionary origins of the genetic code have been proposed. Four themes run through the many hypotheses about the evolution of the genetic code:

Here I summarize the four themes (adapted from wikipedia).

  1. Chemical principles governing specific RNA interaction with amino acids.

  2. Biosynthetic expansion. The standard modern genetic code grew from a simpler earlier code through a process of "biosynthetic expansion".

  3. Natural selection has led to codon assignments of the genetic code that minimize the effects of mutations.

  4. Information channels: Information-theoretic approaches models suggest that the genetic code originated as a result of the interplay of the three conflicting evolutionary forces: the needs for diverse amino-acids, for error-tolerance and for minimal cost of resources.

Other models combine some of these hypothesis.

Selected references:

  1. The genetic code is one in a million. Freeland SJ, Hurst LD. J Mol Evol. 1998 Sep;47(3):238-48.
  2. Selection, history and chemistry: the three faces of the genetic code. Knight RD, Freeland SJ, Landweber LF. Trends Biochem Sci. 1999 Jun;24(6):241-7. Review.
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  • $\begingroup$ I see your point and have read some articles in this direction too. So as you say rare amino acids like Trp have less codons. But why a certain codon came to represent an amino acid is in my opinion a chance event. Just like any evolutionary process $\endgroup$
    – WYSIWYG
    Commented Oct 7, 2014 at 17:44
  • $\begingroup$ So it sounds like there are a bunch of theories on how this codon to amino acid mapping came about - but none of these are supported by specific evidence - its all speculation. Is that correct? $\endgroup$
    – Michael Scott
    Commented Oct 9, 2014 at 2:54
  • $\begingroup$ @WYSIWYG, yes, evolution is random. But here the point is that some outcomes are more advantageous than others. In this case, the assignment of codons to amino acids seem to be an advantageous one (most likely not the optimal one) based on several considerations. $\endgroup$
    – ddiez
    Commented Oct 9, 2014 at 16:28
  • $\begingroup$ @MichaelScott, the many research papers on this topic present supportive evidence. As with any research trying to demonstrate the past, it is very difficult to find irrefutable evidence. $\endgroup$
    – ddiez
    Commented Oct 9, 2014 at 16:34

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