Biology Reference
In-Depth Information
Stimulated by the success in the autopoietic self-reproduction of vesicles, the next step was
to create more complex systems in order to achieve living-like functions in vesicle-based
systems. Due to the structural similarity between lipid vesicles and natural cells, the goal
became the laboratory construction of synthetic minimal autopoietic cells (as a model of
primitive minimal autopoietic cells), in order to understand what could happen when lipid
vesicles hosted simple and complex biochemical reactions. Clearly, in order to build a
model of primitive minimal autopoietic cells primitive molecules are needed.
Unfortunately, our knowledge of which are the primitive molecules that give rise to living
cells is still very poor, and this approach cannot be followed experimentally. What can be
done, however, consists in the use of currently available molecules, such as DNA, RNA,
ribosomes, enzymes, and lipids to try to construct a
minimal autopoietic cell
(SSMC) (
Fig. 14.1b
). As we have highlighted above, autopoiesis resides in the organization
of a network of transformations, not in the chemical nature of its components. This means
that in order to demonstrate experimentally the transition from nonliving separated
molecules to a living autopoietic system it is not important which molecules are used. In
other words, SSMCs can serve as a model of primitive cells only from the viewpoint of
functional organization, not from the viewpoint of molecular structures. Despite the loss of
some
semisynthetic
'
'
aspects, SSMCs are certainly an interesting tool for origin of life
research. Moreover, it is already possible to glimpse some possible future applications in
biotechnology, as we will discuss in the concluding remarks.
primitiveness
'
'
RECONSTRUCTION OF GENETIC/METABOLIC PROCESSES
IN SEMISYNTHETIC MINIMAL CELLS
In order to construct an autopoietic minimal cell in the laboratory using the semisynthetic
approach, as we have seen in the previous section, one has to design and realize a genetic/
metabolic network that gives as output all its components (including the boundary).
In turn, these components will again give rise to the same set of transformation to produce
copies of themselves, and so on. As a result, the minimal cell grows in size, reaches an
unstable physical state, and produces offspring by a fragmentation-like process. What is the
minimal number of molecules needed for accomplishing this task?
264
A starting point for this discussion is given by the studies on
It is well
known that the genome of free-living prokaryotes, like
Escherichia coli
, contains regions that
encode for all proteins and RNAs for its self-maintenance, namely for producing all its
components from simple nutrients that are present in its environment. There are, however,
microorganisms that are symbionts or parasites, and live inside other cells. Evolution has
shaped their genome by eliminating unnecessary genes, so that they have a minimal
genome where only essential genes are kept. This is possible because their
'
minimal genomes.
'
environment
is
'
'
another cell that provides for it the missing biochemicals. By comparing the natural
minimal genomes of several microorganisms, several researchers have tried to answer the
question of what is the universal minimal set of genes for sustaining life in very
permissive conditions (reviewed in
9
). In a recent study, Andres Moya and coworkers
17
have suggested that the minimal genome should contain 206 genes, most of which
(121 genes) are devoted to the transcription/translation and protein processing processes.
Additional genes are needed for DNA processing (16 genes), basal metabolism
(56 genes), and for other cellular processes (13 genes). In principle, therefore, a minimal
autopoietic cell could be built by constructing a cell-like compartment that contains
about 200 genes (those of the minimal genome) and all molecules required for their
expression. Despite the recent advances, however, the experimental approaches that we
will describe below are still too rudimentary to achieve this goal. They have been
developed firstly for understanding each single process separately, and possibly reaching
the final goal of synthesizing a living cell stepwise.
