Such calculations are not only attractive but they are necessary as
well. They create the feeling that the problem of the gene expression at the
cell and organism levels is almost solved. Lots of data however give
grounds to the thought that these are rather hasty and insufficiently
accounted for, since the gene itself proved to be a more complex structure
than expected. To the present day is a question still unanswered: what is a
gene and what are its real dimensions?
It can be definitely stated that there is not a precise and undisputed
definition of the gene, although it underlies the basis of genetics and
molecular biology. As most acceptable is to think that the gene represents
a segment of the chromosome encoding for a functionally active product
(either RNA or the product of its translation — a polypeptide).
The idea of the gene has been changing depending on the level of
knowledge in that field. Let us follow its development in an historical aspect.
Mendel’s experiments binding the inheritance of the features in the
offspring to the hereditary factors bore the term “gene” later on. T. Morgan and
his collaborators have arrived at the conclusion that the gene is an indivisible
structure which is the unit for function, mutation and recombination. N. Dubinin
has launched the idea that the gene can be divided.
If the functions of the genes are judged by the results of their
expression, i.e. by the determination of the hereditary feature, then the
initial definition of classical genetics one gene — one hereditary feature
was rather convincing. When it was established that the gene (cistron) is
divisible into smaller subunit (sites) that can change independently one of
another and mutate at various frequencies (Benzer, 1955—61), two or
more complementary genes can take part in the formation of one and the
same feature (multiple allelism) and only one gene can determine various
features and properties (pleiotropic action of the genes), then its initial
definition proved to be rather limited so that it could meet the more recent
ideas about its structural organization and functions.
The studies of Beadle and Tatum (1941) on the auxotrophic mutants in
Neurospora crasa showed that they are obtained as a result of the
disorders in the synthesis of a given enzyme controlled by a given gene.
This fact gave the authors the grounds to make the generalization one
gene — one enzyme. For the first time the link between genes and
enzymes was established.
Since all enzymes are proteins this definition was modified into “one
gene — one protein”. Based on the achievements of modern molecular
biology and genetics, that most protein molecules are built from several
polypeptide chains whose structure is determined by different genes the up-to-date
formulation of gene expression is one gene — one polypeptide chain.