5.3 A ife-Specific aster Functions: Building p a Progeny 855.3 A Life-Specific Master Function: Building Up a Progeny
Life perpetuates itself. A sterile organism may still be alive, but it misses a key
property of life in that it does not have a progeny. Indeed, its very existence is
simply borrowing time: maintenance of a machine linked to a program, both
doomed to age and die, can hardly allow long‐term survival in an ever‐changing
environment (for a discussion, see [28] and references therein). Some animal
societies have classes of sterile individuals, but they are always firmly connected
to a fertile lineage. If life were only composed of infertile individuals, it would
already be extinct, unless there existed a steady and speedy process of spontane
ous generation with a creation time shorter than the life‐span of individual
organisms. This is more than unlikely and does not, anyhow, fit with the chemis
try of life as we know it. We will therefore accept that life is tightly coupled to the
making of a (young) progeny.
Considering this process, we can see that the ultimate destination of the genetic
program is to make a copy of itself within a copy of the machine that runs the
program. “Copy” here must be defined. How are the processes of program copy
ing and that of cell copying linked together? Remarkably, the actual concrete
copying process differs whether dealing with the program, or with the machine:
the program is replicated in most of the cell’s progeny (i.e., it makes exact copies
of itself ), while the machine’s future is much sloppier, wherein it is only repro-
duced (i.e., it makes similar copies of itself ) [27, 29]. To this dichotomy two time
scales are associated: replication is trustworthy for many generations, while
reproduction makes copies that vary rapidly over time. Genome transplantation
experiments, such as those using synthetic genomes [25], give us a vivid illustra
tion of this functional dichotomy. Extracted at the end of the experiment and
sequenced, the synthetic genome of the bacteria in the recovered colonies is
identical to that which has been transplanted in the host. By contrast, the
machinery, and even the cell’s shape, differs in the initial host and in the cells
making the final colonies (Figure 5.2). In terms of engineering, this is somewhat
unusual, although we all know of man‐made devices that have been progressively
modified, as was Theseus boat (that did not keep a single original of its boards
after some time [11]). The parent machine has aged, and its components have
been replaced by new ones. In the transplantation experiment, this regeneration
process required the use of a new program, differing from the parent one that
had been destroyed, thanks to an astute genetic design [21]. As a consequence,
during multiplication, the program that was used is that of the transplanted
genome, directing the synthesis of entities that differ from those of the initial
host machine.
This state of affairs is far more general than that in the transplantation experi
ment: as in any life form, the components of any SynBio construct age and are
replaced; in parallel, the environment changes and some components are no
longer required and are diluted out while others are expressed. In short, while
the program may remain the same, the machine that runs it is quite variable. It
keeps however its main functional (abstract) properties: reading and expressing
the program, and directing the construction of a progeny, while monitoring the
state of the environment, extracting proper resources and discarding useless or