KEY TERMS:
- Positional information describes the localization of macromolecules at particular places in an embryo. The localization may itself be a form of information that is inherited.
- The definition of the gene has reversed from "one gene : one protein" to "one protein : one gene".
- Positional information is also important in development.
The concept of the gene has evolved significantly in the
past few years. The question of what's in a name is especially appropriate for
the gene. We can no longer say that a gene is a sequence of DNA that
continuously and uniquely codes for a particular protein. In situations in which
a stretch of DNA is responsible for production of one particular protein,
current usage regards the entire sequence of DNA, from the first point
represented in the messenger RNA to the last point corresponding to its end, as
comprising the "gene," exons, introns, and all.
When the sequences representing proteins overlap or have
alternative forms of expression, we may reverse the usual description of the
gene. Instead of saying "one gene-one polypeptide," we may describe the
relationship as "one polypeptide-one gene." So we regard the sequence actually
responsible for production of the polypeptide (including introns as well as
exons) as constituting the gene, while recognizing that from the perspective of
another protein, part of this same sequence also belongs to its gene.
This allows the use of descriptions such as "overlapping" or "alternative"
genes.
We can now see how far we have come from the original one
gene : one enzyme hypothesis. Up to that time, the driving question was the
nature of the gene. Once it was discovered that genes represent proteins, the
paradigm became fixed in the form of the concept that every genetic unit
functions through the synthesis of a particular protein.
This view remains the central paradigm of molecular biology:
a sequence of DNA functions either by directly coding for a particular protein
or by being necessary for the use of an adjacent segment that actually codes for
the protein. How far does this paradigm take us beyond explaining the basic
relationship between genes and proteins?
The development of multicellular organisms rests on the use
of different genes to generate the different cell phenotypes of each tissue. The
expression of genes is determined by a regulatory network that takes the form of
a cascade. Expression of the first set of genes at the start of embryonic
development leads to expression of the genes involved in the next stage of
development, which in turn leads to a further stage, and so on until all the
tissues of the adult are functioning. The molecular nature of this regulatory
network is largely unknown, but we assume that it consists of genes that code
for products (probably protein, perhaps sometimes RNA) that act on other
genes.
While such a series of interactions is almost certainly the
means by which the developmental program is executed, we can ask whether it is
entirely sufficient. One specific question concerns the nature and role of positional information. We know that all parts of a
fertilized egg are not equal; one of the features responsible for development of
different tissue parts from different regions of the egg is location of
information (presumably specific macromolecules) within the cell.
We do not know how these particular regions are formed. But
we may speculate that the existence of positional information in the egg leads
to the differential expression of genes in the cells subsequently formed in
these regions, which leads to the development of the adult organism, which leads
to the development of an egg with the appropriate positional information…
This possibility prompts us to ask whether some information
needed for development of the organism is contained in a form that we cannot
directly attribute to a sequence of DNA (although the expression of particular
sequences may be needed to perpetuate the positional information). Put in a more
general way, we might ask: when we read out the entire sequence of DNA
comprising the genome of some organism and interpret it in terms of proteins and
regulatory regions, could we in principle construct an organism (or even a
single living cell) by controlled expression of the proper genes?
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