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Deciphering the Genetic Code, 1958-1966
Letter from Francis Crick to Severo Ochoa (December 5, 1962)
Thank you for your note of October 22. When the Nobel Prize for Medicine was announced, I was in Cordova, Argentina, and
I was very happy to hear the good news. I had gone to Argentina accompanied by my wife on a short lecture trip. They had
a beautiful spring there and we had a wonderful time.
I also want to thank you for your earlier letter, of September 21, and the preprint of your review on the code for Progress
in Nucleic Acid Research. At that time, we had already analyzed many of the polymers used in our work and I understand that
you have seen the data sent by Lengyel to Bretscher. We have recently obtained exciting new data as we found that poly A stimulates
the incorporation of lysine and appears to direct the synthesis of polylysine. This made it possible to make A-rich non-U
polymers and we got a number of code triplets without U for several amino acids. I am enclosing the preprint of a paper that
we have submitted to the National Academy. It looks therefore as if the code is pretty degenerate although I am not sure
yet whether some kind of doublet code as proposed by Roberts could not account for the results. The enclosed sheet lists
the U- and non-U triplets we have obtained so far and you can see that several of them share doublets. Therefore, in the
case, let us say, of histidine it may be that for AUC and ACC only the AC doublet is meaningful so that a single rather than
a degenerate code is in fact involved. The final criterion for degeneracy of the code for each amino acid might well have
to be based on whether there is one or more transfer RNA's specific for this amino acid.