A layman calls for help!(genetic code)

Daniel Zabetakis dan at cubmol.bio.columbia.edu
Sat May 15 10:47:45 EST 1993

In article <1t0379$h5d at usenet.INS.CWRU.Edu> cu630 at cleveland.Freenet.Edu (Gustave Rabson) writes:
> There are 64 (4 cubed) permutations
>of 4 bases taken 3 at a time, but only 20 combinations.
>That is, if we consider two words to be the same if they
>consist of the same letters in any order there are
>only 20 combinations of the 4 letters. This would fit
>very well with 20 Amino Acids. So my question is
>how firmly established is it that combinations
>like GGU and GUG are different? What about mirror
>images, i.e. GCU and UCG?

    Extremely well established, actually. 
    The way that the 64 permutations are reduced to fit the 20 AA's is
not by combination, but rather by discounting the third nucleotide in the
codon. So CCA, CCU, CCG, CCC all code for proline. There are several other
AA's like this. So the last nucleotide is of reduced significance.
   The mirror image, isn't. It looks mirrored only when you write it as
a single letter triplet. Those two codons look very different chemically.


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