Inference
of the Universal Genetic Code from poly-monomers &
poly-dimers
Nirenberg prepared poly U, C, A, &
G RNA messages: this allowed him to infer that UUU,
CCC, AAA, & GGG coded for F, P K, &
G respectively.
Polymerization of a di-nucleotide yields a polynucleotide
with two alternating
triplets irrespective of the starting point. Thus UC-UC-UC
for example produces a polypeptide with alternative S & L
residues. Note that of the twelve possible di-nucleotides, the
alternative pair with the letters reversed produces the same
alternating peptide.
Use of all possible dinucleotides gives codons for 11 amino acids,
including redundant codons CGC and AGA for R;
note the degeneracy at the 2nd bases is unusual. In
combination with the mono-nucleotide codons, this solves exactly one
codon in each of the boxes defined by the first two
bases in the codon. These experiments would not identify any of
the three stop codons (no experiments should 'fail'
because they incorporate nothing), and the amino acids assigned
codons do not include W Q M N D or the alternative codons
for L' and S'.
