Biology Reference
In-Depth Information
longer linearly and directly dependent on their position in the
DNA, the three-dimensional structure of the chromatin results from
DNA folding, the possibilities for which are always constrained by
the linear structure of the DNA molecules. In three-dimensional
chromatin, the linear distance between the genes therefore remains
an important parameter determining the probabilities of the genes
being expressed.
It is also interesting to note that this model provides an expla-
nation for the fact that more than 95% of DNA seems to have no
function in multicellular organisms. This is made up of sequences
which are often interleaved between the genes and do not corre-
spond either to proteins or to regulator elements. This phenomenon
concerning the amount of DNA has been called the 'C-value
paradox' or more commonly 'junk DNA'. Indeed, in our model all
the sequences, even if they do not code for proteins, have a role:
they determine the relative position of the genes and their proba-
bilities of being expressed during embryogenesis.
DNA therefore continues to exert an important role in onto-
phylogenesis. Its structure is the result of the history of the organ-
ism which influences its immediate development, but the way it
functions does not come into the domain of the theory of genetic
programming. It does not contain a 'plan' of the organism. Nor is
it a self-organisation mechanism. Chromatin structuring is not a
spontaneous phenomenon: it is constrained by cell selection
via
the
signals (or selectors) the cell receives and by the DNA structure
which is itself the result of natural selection. Gene expression is
therefore both intrinsically probabilistic and subject to natural
selection.
To conclude, throughout this chapter we have developed a the-
ory which acknowledges in ontogenesis and phylogenesis a single
process. The theory surmounts the contradiction in genetic deter-
minism by integrating the history of the organism in its ontogenesis,
and it advances new models of cell differentiation and gene expres-
sion. We have also seen that it is compatible with many experi-
mental facts and allows us to make testable predictions. However,
