In this letter Crick refers to genetic experiments undertaken with Leslie Barnett, Sydney Brenner, and Richard Watts-Tobin
on acridine mutants, which indicated that the genetic code was a triplet code. Compelling evidence that poly-U (a synthetic
form of RNA consisting of bases of a single type, uracil) coded for the amino acid phenylalanine was first presented by the
American biochemist Marshall Nirenberg in the summer of 1961.
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1961-10-09 (October 9, 1961)
Sinsheimer, Robert L.
Original Repository: Wellcome Library for the History and Understanding of Medicine. Francis Harry Compton Crick Papers
Many thanks for your letter and the further invitation. I would be most reluctant to come west in February because I have
to be in San Francisco from about 9th to 1lth or so of April, and could perhaps manage to come to give a colloquium at Cal.Tech.
after that and just before I go home. Would that be a suitable time?
We now have convincing genetic evidence that the coding ratio is 3 or a multiple of 3. It is not clear that we can prove
that it is 3 rather than 6, but we are trying. I will send you a copy of a manuscript as soon as we have one. Jim Ofengand
has also confirmed, in a preliminary way, that poly U makes polyphenalamine.