Information Technology Reference
In-Depth Information
most of the compounds they need from simple chemical sources, and appear to
have a simpler structure than eukaryotes (cells with nucleus) and archea. The
ultimate reason for this simplicity is evolution: bacteria have \chosen" to exploit the
speed of replication in spite of complexity, and optimized accordingly their working
machinery. Viruses have progressed a lot in the direction of speed, but they need a
much more structured background: the presence of a chemical machinery assembled
by other living cells.
Bacteria absorb nutrients (energy), amino acids and other chemicals (building
blocks) from the medium, and uses them to increase their size, up to a point in
which they divide in two (or sprout some buddies). These tasks are carried out by
a biochemical network of proteins. Proteins act as enzymes, transforming chem-
ical elements, as structural elements and have also a regulatory functions either
by directly activating or inactivating other proteins, or by promoting or blocking
the production of other or same proteins. In fact, in a growing bacteria, there is
continuously the need of producing new proteins and all the other constituents of
the body.
A protein is synthesized as an one-dimensional chain of amino acid. The tri-
dimensional shape of a protein, and thus its chemical function, is dened (at least
in many simpler cases) by its one-dimensional sequence, that folds and assumes its
working conformation by itself.
a
There are only 20 amino acids used by living be-
ings. The one-dimensional sequence of a protein is coded into the one-dimensional
sequence of basis (gene) in a chromosome. Many bacteria have one large chromo-
some, plus a variable number of smaller ones (plasmids). A chromosome is a double
helix of DNA, that can be view (in computer terms) as a sequence of symbols from
a four-letter alphabet (ATCG). The translation of a gene into a protein takes place
in several steps. First the gene is transcribed into an intermediate form, using RNA.
In the RNA alphabet, thymine (T) is replaced by uracil (U). This messenger RNA
(mRNA) in eukaryotes are further processed by splicing pieces (introns), but the
basic working is similar.
b
Messenger RNA is then translated into a protein by a large complex of proteins
and RNA, called ribosome. A ribosome acts as a catalyst, by allowing other small
RNA-amino acid complexes called tRNA to bind to mRNA. This binding is rather
specic. A tRNA present a triplet of basis that must complement (with some toler-
ance) to the triplet (called codon) in the specic region of mRNA that is processed
by the ribosome. The tRNA carries an amino acid that is specic to its anti-codon
(this pairing is performed by other enzymes). Thus, by this large biochemical net-
work, we have the production of a protein following the information stored in a
gene.
a
Many larger complexes are formed by more that just one protein, and some protein need the help
of other enzymes to stabilize the three-dimensional shape with covalent bonds or to add metals
ions, sugar chains and other elements.
b
Actually, in archea, too, there are introns, and also in bacteria self-splicing portions of RNA are
present.
