Information Technology Reference
In-Depth Information
When a cell has grown suciently, it divides. In order to perform this step,
also the DNA has to be replicated, using again other proteins and RNA fragments.
Actually, in growing bacteria, both processes (production of body material and
DNA copying) are performed in parallel, so that some part of the chromosome can
be present in multiple copies. The splitting of a cell into two can also be seen
as balance between the exponential growth of the body and the (almost) linear
copying of the DNA. All life long, but especially during the copy phase, mutations
may occur, changing some basis, inserting or deleting others, etc. Mutations are
essential to generate diversity, but, as we shall see, they need to occur quite rarely.
Clearly, in multi-cellular organisms, only mutations that occur in the germinal line
are transmitted to the progeny.
The translation of a gene depends by other proteins: they can block the trans-
lation by binding themselves to DNA, or by altering the DNA itself (methylation).
Other proteins are also needed to promote the transcription of a gene. Thus, from
the proteins' point of view, a gene is just a way of producing other proteins, un-
der the inuence of the specic pool of proteins that are working together into the
organism.
Not all genes are translated into proteins. For instance, tRNA genes are only
transcribed into RNA. Other portions of DNA are never transcribed, but can alter
the biochemistry of the cell. For instance, specic sequences may inhibit the expres-
sion of genes by bending the DNA, or preventing the binding of promotor proteins.
Other DNA sequences may interfere with the process of DNA copying, for instance
by increasing the number of repeats of themselves (microsatellites, ALU). Other
sequences may jump from a position of the chromosome to another (transposons),
eventually carrying some portion of DNA with itself. By this mechanism they can
inactivate, activate or merge genes.
Bacteria can also alter their genetic contents by capturing or exchanging pieces
of DNA (plasmids), a process that resemble the sexual reproduction of some eu-
karyotes.
Finally, retroviruses can alter the genome of the host by inserting their own
code (or a precursor of it in RNA retroviruses). This process, that reverts the
usual information path from DNA to RNA and proteins (the so-called \central
dogma" of biology), is reputed to be one of the most powerful mechanisms of genetic
transformations. Transposons are probably the relics of viruses, and a substantial
part of some eukaryotic genome can be recognised as inactivated viruses.
The last important element is selection. In our example, bacteria living in a test
tube will proliferate, duplicate and probably dierentiate. Sooner or later, they will
consume all the nutrients in the solution. A few of them can survive sometime by
\eating" others, but clearly, in a closed environment, the last fate is the extinction
of all living forms.
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Life (replicators) needs an open environment, with uxes of
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Actually, this can take a long time, since many bacteria are able to \freeze" themselves in form
of spores, that can survive for a long time in rather harsh environments.
