Self and the Beginning of Life 55“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityimprecise and highly “wobble”. Thus, amino acids of related properties
might substitute for each other, and the proteins might function enzy-
matically in a statistical rather than discrete manner.
The primitive tRNA might serve as its own amino acid activating
enzyme, before the advent of aminoacyl-tRNA synthetase (AARS).^64
Along this line of thinking is the hypothesis of “proto-anticodon RNA,”
which posits that amino acids stereochemically bind to their respective
anticodons that are nested in the proto-tRNA; the proto-tRNA presum-
ably serves as an enzyme catalyzing the aminoacylation of its own 3’ end,
forming aminoacylated proto-tRNA. This hypothesis attempts to explain
(1) the origin of the genetic code, and (2) why only the “L” form of
amino acids is incorporated into proteins. (The latter is due to the ori-
entation of the amino acid side chains with respect to the space created
by the anticodon and its adjacent nucleotide structure, allowing only the
L-amino acids but not their “D” counterparts to fit in.) This hypothe-
sis places anticodons ahead of codons, both preceding the translation
mechanism.^65
3.18 A Recapitulation: What does a Genetic Code Mean?
A code, like any symbol, has no meaning unless it signifies something. For
those who subscribe to the RNA World hypothesis, it naturally follows
that the information-laden RNAs (equivalent to the present day mRNA)
existed before the codons were translated to the amino-acid sequence of
proteins. However, as I argued above, a nucleotide sequence meaningful
to an RNA World does not make metabolic sense when translated into a
protein. It is more reasonable to believe that the genetic code appeared
as a way to preserve the information derived from primitive proteins that
were metabolically functioning. The primitive mRNA thus formed could
double as a gene by undergoing replication by complementarily, either
by autocatalysis, or by the action of ribozyme or protein enzyme.
The final step in the origin of life would be the deposition of
genetic information in DNA molecules. DNA is superior to RNA for this