Shortly after Cech and Altman discovered the catalytic properties of
RNA, the ideal of an RNA world in which early life consisted on self-
replicating RNA molecules was first proposed by Wallace Gilbert (Gilbert
1986). One of the earliest stages of evolution was the formation of
catalytic RNA in the primordial soup of the primitive Earth. The amount
of catalytic RNA would have increased exponentially with variants
providing the opportunity for the selection of ribozymes with enhanced
catalytic ability. In the RNA world hypothesis, the division of function
into genetic information storage with DNA and catalysis by proteins
would have arisen at a later stage of evolution. Proteins would have arisen
after variants of self-replicating RNA molecules developed the ability to
catalyze the condensation of amino acids into peptides. Some time after
the development of a primitive protein-synthesis system, DNA mole-
cules would have developed with sequences that were complementary
to those of the self-replicating RNA. Eventually, DNA took over the role
of conserving the genetic information and RNA became an intermediate
in the process of converting information stored in DNA into the produc-
tion of proteins. Despite significant issues concerning the RNA world, such
as the stability of RNA and the lack of detailed mechanisms, the idea of
the RNA world continues to be embraced by scientists and serves as a
platform for research (Orgel 1998; Huttenhofer & Schattner 2006).160 PARTI THERMODYNAMICS AND KINETICS
Figure 7.20Certain
virus-like elements
called virusoids have
small segments
called hammerhead
ribozymes due
to the shape of
their secondary
structure. These
RNA segments fold
into well-defined
three-dimensional
structures.