Self and the Beginning of Life 45“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanitythe spontaneous generation of a self-replicating ribozyme has not been
proved.
Setting aside the issue of replication, other artificial ribozymes
have been produced to mimic a variety of unexpected enzyme functions.
Again, there is no evidence that such ribozymes actually exist in nature,
today or in ancient times. We should be cautious of the fact that the pres-
ence of a man-made compound or a man-made enzyme does not entail
its presence in nature. For example, protein enzymes have been artifi-
cially produced that perform much more efficiently than their natural
counterparts, although such artificial molecules do not occur in nature.
3.11 Evidence against RNA: (III) How did an RNA World
Turn Over to a Protein World?
Perhaps the greatest conceptual hurdle encountered in formulating an
RNA-dominated cell is how to visualize a smooth transition from an
RNA-based to a protein-based metabolism without seriously disrupting
the cellular order.
Theoretically, in a fully developed RNA World, there should be
two pools of RNA: a gene-like RNA pool for information transfer and
an enzymatic RNA (ribozyme) pool for metabolic function.^22 Further-
more, the 3-D conformation of the ribozyme, which is critical for its
catalytic activity, is dictated by its nucleotide sequence, which in turn
depends on sequence information contained in the genomic RNA. But
once the enzymatic RNA was replaced by a protein, the information
stored in the genomic RNA, which originally translated into a func-
tioning ribozyme, no longer translated into a protein whose amino acid
sequence formed a 3-D structure suitable for enzyme action. Thus, the
transition from a well-formed RNA World to a Protein World as we see it
today would amount to a major overhaul of the entire genetic and meta-
bolic systems, possibly incurring deleterious if not devastating outcomes.
This applies even to a minimally functioning “ribocyte” (a hypothetical
RNA- dominated cell), which must contain at a minimum 100 genes of at