Essential Cell Biology 5th edition

RNA and the Origins of Life
259
as we will discuss thoroughly in the next chapter, the initiation of transcription
is the most common point for a cell to regulate the expression
of its genes.
RNA AND THE ORIGINS OF LIFE
The central dogma—that DNA makes RNA, which makes protein—presented
evolutionary biologists with a knotty puzzle: if nucleic acids are
required to direct the synthesis of proteins, and proteins are required to
synthesize nucleic acids, how could this system of interdependent components
have arisen? The prevailing view is that an RNA world existed
on Earth before cells containing DNA and proteins appeared. According
to this hypothesis, RNA—which today serves largely as an intermediate
between genes and proteins—both stored genetic information and catalyzed
chemical reactions in primitive cells. Only later in evolutionary time
did DNA take over as the genetic material and proteins become the major
catalysts and structural components of cells (Figure 7–47). As we have
seen, RNA still catalyzes several fundamental reactions in modern cells,
including protein synthesis and RNA splicing. These ribozymes are like
molecular fossils, holdovers from an earlier RNA world.
Life Requires Autocatalysis
The origin of life requires molecules that possess, if only to a small extent,
one crucial property: the ability to catalyze reactions that lead—directly
or indirectly—to the production of more molecules like themselves.
Catalysts with this self-reproducing property, once they had arisen by
chance, would divert raw materials from the production of other substances
to make more of themselves. In this way, one can envisage the
gradual development of an increasingly complex chemical system of
organic monomers and polymers that function together to generate more
molecules of the same types, fueled by a supply of simple raw materials
in the primitive environment on Earth. Such an autocatalytic system
would have many of the properties we think of as characteristic of living
matter: the system would contain a far-from-random selection of interacting
molecules; it would tend to reproduce itself; it would compete with
other systems dependent on the same raw materials; and, if deprived of
its raw materials or maintained at a temperature that upset the balance
of reaction rates, it would decay toward chemical equilibrium and “die.”
But what molecules could have had such autocatalytic properties? In
present-day living cells, the most versatile catalysts are proteins, which
are able to adopt diverse three-dimensional forms that bristle with chemically
reactive sites on their surface. However, there is no known way in
which a protein can reproduce itself directly. RNA molecules, by contrast,
possess properties that—at least, in principle—could be exploited to catalyze
their own synthesis.
Big Bang
solar
system
formed
first cells
with DNA
first
mammals
present
10
time (billions of years ago)
5
14
RNA
WORLD
Figure 7–47 An RNA world may have existed before modern cells arose.