Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE RNA WORLD AND THE ORIGINS OF LIFE
365
catalysis
Figure 6–92 An RNA molecule that
can catalyze its own synthesis. This
hypothetical process would require
catalysis both of the production of a
second RNA strand of complementary
nucleotide sequence (not shown) and the
use of this second RNA molecule as a
template to form many molecules of RNA
with the original sequence. The red rays
represent the active site of this hypothetical
RNA enzyme.
But the efficient synthesis of RNA by such complementary templating mechanisms
requires catalysts to promote the polymerization reaction: without catalysts,
polymer formation is slow, error-prone, and inefficient.
Because RNA has all the properties required of a molecule that could catalyze
a variety of chemical reactions, including those that lead to its own synthesis (Figure
6–92), it has been proposed that RNAs served long ago as the catalysts for template-dependent
RNA synthesis. Although self-replicating systems of RNA molecules
have not been found in nature, scientists have made significant progress
toward constructing them in the laboratory. While such demonstrations would
not prove that self-replicating RNA molecules were central to the origin of life on
Earth, they would establish that such a scenario is plausible.
MBoC6 m6.99/6.92
How Did Protein Synthesis Evolve?
The molecular processes underlying protein synthesis in present-day cells seem
inextricably complex. Although we understand most of them, they do not make
conceptual sense in the way that DNA transcription, DNA repair, and DNA replication
do. It is especially difficult to imagine how protein synthesis evolved
because it is now performed by a complex interlocking system of protein and RNA
molecules; obviously the proteins could not have existed until an early version of
the translation apparatus was already in place. As attractive as the RNA world idea
is for envisioning early life, it does not explain how the modern-day system of protein
synthesis arose. Although we can only speculate on the origins of the genetic
code, several experimental observations have provided plausible scenarios.
In modern cells, some short peptides (such as antibiotics) are synthesized
without the ribosome; peptide synthetase enzymes assemble these peptides, with
their proper sequence of amino acids, without mRNAs to guide their synthesis. It
is plausible that this noncoded, primitive version of protein synthesis first developed
in the RNA world, where it would have been catalyzed by RNA molecules.
This idea presents no conceptual difficulties because, as we have seen, rRNA catalyzes
peptide bond formation in present-day cells. However, it leaves unexplained
how the genetic code—which lies at the core of protein synthesis in today’s cells—
might have arisen. We know that ribozymes created in the laboratory can perform
specific aminoacylation reactions; that is, they can match specific amino acids to
specific tRNAs. It is therefore possible that tRNA-like adaptors, each matched to a
specific amino acid, could have arisen in the RNA world, marking the beginnings
of a genetic code.
Once coded protein synthesis evolved, the transition to a protein-dominated
world could proceed, with proteins eventually taking over the majority of catalytic
and structural tasks because of their greater versatility, with 20 rather than 4 different
subunits. Although these ideas are highly speculative, they are consistent
with the known properties of RNA and protein molecules.
All Present-Day Cells Use DNA as Their Hereditary Material
If the evolutionary speculations embodied in the RNA world hypothesis are
correct, early cells would have differed fundamentally from the cells we know
today in having their hereditary information stored in RNA rather than in DNA
(Figure 6–93). Evidence that RNA arose before DNA in evolution can be found
RNA-based systems
DNA
RNA
EVOLUTION OF RNAs THAT
CAN DIRECT PROTEIN SYNTHESIS
RNA and protein-based systems
RNA
EVOLUTION OF NEW ENZYMES
THAT REPLICATE DNA AND
MAKE RNA COPIES FROM IT
present-day cells
RNA
protein
protein
Figure 6–93 The hypothesis that RNA
preceded DNA and proteins in evolution.
In the earliest cells, RNA molecules (or their
close analogs) would have had combined
genetic, structural, and catalytic functions.
In present-day cells, DNA is the repository
of genetic information, and proteins
perform the vast majority of catalytic
functions in cells. RNA primarily functions
MBoC6 m6.110/6.93
today as a go-between in protein synthesis,
although it remains a catalyst for a small
number of crucial reactions.