Page 200 - Introduction to Paleobiology and The Fossil Record

THE ORIGIN OF LIFE 187
self-replicating
vesicle
replicase
protocell
linking function
(e.g. ribozyme)
cell
Figure 8.2 The model behind “RNA world”,
where an RNA replicase and a self-replicating
membrane-bound vesicle combine to form a
protocell. Inside the vesicle, the RNA replicase
functions, and might add a function to improve
the production of the vesicle wall through a
ribozyme. At this point, the RNA replicase and
the vesicle are functioning together, and the
protocell has become a living cell, capable of
nutrition, growth, reproduction and evolution.
Read a general introduction to RNA world at
http://www.blackwellpublishing.com/
paleobiology/. (Based on information in Szostak
et al. 2001.)
molecule cannot both replicate and trigger
that replication. The minimum requirement is
that two RNA molecules interact, one to act
as the enzyme to bring together the compo-
nents, and the other to act as the gene/tem-
plate. Together the template and the enzyme
RNA combine as an RNA replicase. But these
components have to be kept together inside
some form of compartment or cell, otherwise
they would only occasionally come into
contact to work together. Szostak and
colleagues then proposed there must be a
second precellular structure they call a self-
replicating vesicle, a membrane-bound struc-
ture composed mainly of lipids (organic
compounds that are not soluble in water,
including fats) that self-replicates, or grows
and divides from time to time. The RNA rep-
licase at some point entered a self-replicating
vesicle, and this allowed the RNA replicase to
function effi ciently.
This is a protocell, but it is not yet living.
It is just a self-replicating membrane bag with
an independent self-replicating molecule
inside. To make the protocell function as an
integrated cell, the RNA replicase has to carry
out a function that benefi ts the membrane
component. For example, the RNA repli-
case might generate lipids for the membrane
through the medium of a ribozyme. With the
membrane keeping the RNA replicase together
and so improving its function, and the RNA
replicase producing lipids for the membrane,
the protocell has become a cell. The two func-
tions are coupled, and the cell can evolve, as
vesicles with improved ribozymes can grow
and divide, and become more abundant than
others. So, we have life and we have evolu-
tion. The cell is alive because it has the ability
to feed itself, to grow and to replicate. Evolu-
tion can happen because the cells show dif-
ferential survival (“survival of the fi ttest”),
and the genetic information for replication is
coded in the RNA.
A number of researchers have carried out
experiments to explore all these steps in the
hypothetical RNA world model. They have
succeeded in evolving ribozymes capable of a
broad class of catalytic reactions, including
linking components of RNA and lipid mole-
cules, and over time the molecules are selected
to perform more efficiently. Much work has
yet to be done to show how the whole process
could have worked, especially to improve the
efficiency and accuracy of copying from the
template. The other aspect of the model is
the self-replicating vesicle. Experiments here
have focused on simple physical models for
how oily droplets might incorporate free-
floating lipids, and so grow, and then how the
droplets or vesicles might divide when they
reach a certain size or when external forces
are applied, perhaps by the movement of