Biology Reference
In-Depth Information
Causality
Structure
Function
Selection
Fig. 12.14 The duality of the structure-function relation in biology or the cyclic relation between
structure and function
1
2
3
Genes
(or
DNA
)
RNA
Proteins
O
rganized
P
hysico
c
hemical
P
rocesses
(
OPCPs
)
4
Cell Functions
Fig. 12.15 Molecular mechanisms underlying genotype-phenotype coupling. 1
¼
transcription;
2
¼
translation; 3
¼
enzymic catalysis; 4
¼
Output mechanisms (e.g., secretion, chemotaxis, cell
dissipation of free energy (see Sect.
6.2.11
). So to completely understand how the
human genome functions, it is necessary to elucidate how
free energy
derived from
chemical reactions (e.g., oxidation of glucose to CO
2
and water, ATP hydrolysis) is
combined with
information
encoded in DNA to effectuate various biological
functions of the cell. In this view, the next major step in the Human Genome Project
must include a complete elucidation of the molecular mechanisms underlying
genotype-phenotype coupling
.
It is a truism to state that
structures determine functions
over the ontogenetic
time scale (seconds to years), but
functions select structures
over the phylogenetic
time scale (decades to billions of years). We may refer to this fact as
the duality of
structure-function relations in biology, or the cyclic relation between structure and
function in biology
which may be represented schematically as shown in Fig.
12.14
:
How functions select structures seems well understood in terms of the current
evolutionary theories rooted in the environment-initiated selection of the fittest
reproducing systems among varieties of organisms made available by mutations
and other novelty-generating mechanisms. However, the molecular mechanisms
underlying the causal relation between structure and function on the molecular
level, i.e., the problem of
genotype-phenotype coupling
, or the question as to how
genes control cell functions, are as yet poorly understood.
During the past century, we have learned a great deal about how genes control
cell functions, which may be summarized as shown in Fig.
12.15
:
The overall processes of the genotype-phenotype coupling can be divided into
four distinct subprocesses (Fig.
12.15
). Of these four, the first three processes
