Biology Reference
In-Depth Information
This was the extensive homology of organisms in terms of their intracellular
organization, as well as in terms of the amino acid sequences of their correspond-
ing proteins. In principle, the major food substance glucose could be oxidized
in many ways to carbon dioxide with the harvest of much of the corresponding
free energy. Virtually all organisms, however, possess the glycolytic pathway
and the tricarboxylic acid cycle, and many contain the membrane-associated
electron-transfer chain, which comprise one way of accomplishing this overall
process.
A fortiori
, the enzyme that catalyses the phosphorylation of glucose by
ATP, is sufficiently homologous also in terms of its amino acid sequence, for its
sequence to be identified in many newly sequenced genomes, through the sophis-
ticated techniques of bioinformatics. Even more strongly so, functional domains
of proteins (such as ATP binding sites) have been sufficiently conserved through
evolution to be recognized between genomes. On another planet with perhaps
much higher rates of net mutagenesis, and much lower selection pressure, this
may be different, but for our planet this phenomenon of extensive homology has
been an enormous asset to molecular biology. To many newly sequenced genes,
a function is assigned simply on the basis of homology of sequence, and in
many cases this assignment turns out to be correct, qualitatively. An important
consequence is also that the phrase 'understanding life' does have a meaning.
It could have been such that molecules, mechanisms and pathways differed
immensely between organisms and that each organism had solved the problem
of how to stay alive in its own, entirely different way. It is quite clear now that
this is not the case; life as we know it in a broad sense is probably maintained
in just one single way, with 'minor' variations on the theme. This is not to say
that this variation, which is minor in terms of principle and quality, is not vast
in terms of quantity. Biological diversity especially in the microbial realm is
enormous. Accordingly, life is able to maintain itself under a very wide variety
of conditions on this planet, but again, essentially through extensive variation
on a single theme. Of course, this greatly motivates the scientific question of
what constitutes this essentially uniform molecular basis of life.
The maps and structures of living cells, i.e. the field that may be called cell
biology, were considered known in the 1980s in their essence. What was lacking
was the completeness. Although for each type of network, a number of examples
had been well documented, many actual networks had not yet been identified.
More disturbingly, however, every now and then a cellular component was
discovered that was strongly involved in the already 'known' pathways, most
often in their regulation, but often even in their mechanism. Examples included
fructose 2,6 bisphosphate in glycolysis, the chaperonins in the protein synthesis
pathway and ubiquitinylation in signal transduction. In addition, although some
cellular behaviour could be explained qualitatively on the basis of the known
networks, much other behaviour was in conflict with what was known, or simply
not explained by it. The conflicts could not be used constructively as falsifications
