52 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”But since there are 20 different kinds of amino acids, if we were to ran-
domly associate all the amino acids with all the available codons (giving
allowance for one stop codon; a stop codon is like a “punctuation mark”
at the end of a protein “sentence”), we would end up with approximately
1 × 1084 different versions of the coding system (derived from 21^64 ). That
means for two independently arisen genetic code systems to be identi-
cal, the chance is one in 10^84 , a virtual impossibility.^59 The fact that all
extant organisms on Earth use the same standard genetic code indicates
that the choice cannot be random. This, along with the unity of energy
utilization and biosynthetic pathways, provides a strong argument for a
common ancestry for all lives on Earth. Alternatively, if life sprang up
from more than one site, there must be certain deterministic factors
involved in the process.
The following information suggests the presence of determinis-
tic factors in the assignment of the genetic code: (1) Codons for the
same amino acids, or for amino acids with similar physical properties,
are grouped together so that a mutation by changing one nucleotide is
less likely to alter the property of the protein. For example, multiple
codons for the same amino acid all have similar first two nucleotides;
codons for aspartic acid and glutamic acid differ by only one nucleo-
tide (the last); amino acids with a hydrophobic side chain all have the
nucleotide U in the middle of the codon. (2) Amino acids that share
the same synthetic pathway tend to have the same first nucleotide in
their codons.^60 (3) The four amino acids that are the most abundant in
meteorites and in Miller’s prebiotic synthetic experiments — glycine,
alanine, valine and aspartic acid — are coded by the codons GCC, GGC,
GUC and GAC, respectively, the only variation being in the second let-
ter, implicating that the primitive codon started with the triplet GXC,
with X being one of the four nucleotides.^61 (4) Experiments on in vitro
RNA-amino acid binding found that, for the majority of amino acids, the
RNA binding site usually contains either a codon or an anticodon cor-
responding to that amino acid.^62 Based on this finding, Yarus proposed a
stereochemical hypothesis for the origin of the genetic code. He posited