Information Technology Reference
In-Depth Information
start
stop
AUG AAA CGA UUC AGU GAU GGC UGA
CCG
mRNA
Protein
M
K
R
F
S
DG
Figure 1.7.
An oversimplification of translation, showing the essential elements:
the mRNA template, the start and stop codons, a charged tRNA, and the emergent
protein chain. The sequence shown here is unrealistically short to display both the
start and stop codons.
computer system as such, the rules of translation (in the broader sense) are
simply defined and simply applied: we don't have to deal with chemistry.
Indeed, we don't have to deal with either intermediate transcription proc-
esses or with complicated genetic codes requiring complicated translation
mechanisms. The genetic code of gene expression programming is a simple
one-to-one relationship between the symbols of the genome and the functions
and variables (also called terminals or leaves in the jargon of evolutionary
computation) they represent.
Posttranslational Modifications
When the machinery of translation reaches a stop signal, a non-functional
protein chain is released. Immediately after their release, protein chains are
subjected to a variety of modifications. The first of these so called
posttranslational modifications is common to all proteins and consists in the
folding of the protein chain in its unique three-dimensional structure. Some
proteins are further subjected to other posttranslational modifications like,
for instance, the chemical modification of some amino acids which greatly
enriches the language of proteins; the formation of covalent bonds between
particular amino acids; and the removal of some fragments to shorten the
chain length. Finally, some folded protein chains (subunits) must aggregate
with other subunits to form a multi-subunit protein. Such multi-subunit pro-
teins include many of the most important enzymes and transport proteins in
the cell. These proteins are said to have a quaternary structure, the highest
level of protein organization.
What is important to understand here, is that each level of protein organi-
zation is built on the lower levels, and everything is dictated by the primary
structure of the protein, which obviously is ultimately dictated by the gene.
Another important thing is that when a protein chain folds itself, amino acids
separated by long stretches in the linear chain might be brought together in
