Self and the Beginning of Life 53“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanitythat the current triplet genetic code is a vestige of the amino acid-tRNA
affinity in ancient times. The question arises as to why stereo chemical
interactions no longer apply to the modern tRNA. Yarus explained this
with another hypothesis, called “escaped triplet hypothesis,” which says
that in later days the original binding triplets were relocated to become
codons on mRNAs, and anticodons on tRNAs.^63 How the relocation took
place has not been explained.
It would be impossible to consider the origin of the genetic code
without also tackling the overall translation mechanism — the con-
version of the code from the RNA language to the protein language.
In modern cell biology, there are four major players in this information
transfer: (1) the activated free amino acids (to be incorporated into pro-
teins), (2) the transfer RNA (tRNA) that contains the anticodon, (3) the
messenger RNA (mRNA) with its three-letter codons, (4) the protein
enzyme aminoacyl-tRNA synthetase (AARS). (Peptidyl transferase is
part of the ribosome, but it is not involved in message interpretation.)
tRNAs are adapters that match the cognate amino acids to the three-
letter genetic codes on the mRNA. This is accomplished in two steps.
The first is the attachment of amino acids to the correct tRNAs, aided
by aminoacyl-tRNA synthetase. The amino acid is hooked to one end
(the 3′ end) of the tRNA molecule with a covalent bond, while near the
middle of the tRNA is situated the anticodon (a triplet-nucleotide), to
be bound to the complementary codon on the mRNA in the next step.
AARS is unique in that it serves as an interpreter that “speaks two lan-
guages” — RNA and protein. The enzyme is able to do this by possessing
dual recognition sites (see Fig. 3.4A), one for the amino acid and the
other for the anticodon on the tRNA. AARS is a family of 20 different
proteins with considerable structural diversity, each recognizing a par-
ticular amino acid; but because of degeneracy of the genetic code, each
amino acid may correspond to more than one codon, and thus requires
more than one cognate tRNA. In fact, there are 61 different tRNAs, cor-
responding to the number of available genetic codons (64 minus 3 stop
codons in humans), whereas there are only 20 AARS corresponding to