repeated and unique fractions there are intermediate fractions as well. The
specificity of genome organization in eukaryotes is reviewed in detail by
Ryskov (1989).
The repeated nucleotide sequences were discovered by Britten and
Kohne (1968). Though to a great extent arbitrarily, they are subdivided
into manifold and moderately repeated ones. It is believed that the
intermediate fraction consists of a number of families, one of which being
the satellite DNA. Their biological role and importance in the
organization of the genome remain unclear.
The transfer of genetic information from RNA to DNA due to the
reverse transcriptase proved a sensational interest. Its experimental
proof started with the publications of Temin (1960, 1964 a, b) on some
oncogenic viruses. In only a few years time it was confirmed by other
authors (Spiegelman et al., 1965, 1970 a, b; Baltimore, 1970; Temin,
Mizutani, 1970; Temin, Baltimore, 1972, etc.).
In the beginning the reverse transcriptase was met with a wide
distrust. It was considered impossible, erroneous or an isolated case in
certain oncogenic viruses. After that the scope of the studies was
broadened and it was confirmed on non-oncogenic viruses, cancer
tissues and intact cells including cells of higher organisms. At present
reverse transcriptase is well-established fact in science.
Normile’s report (1996) on the hypothesis of the Japanese scientists
M. Furusawa and H. Doi is now arousing similar surge of interest. They
have suppose that both DNA chains replicate separately from one
another. The leading chain is constantly synthesized in the direction in
which the helix is uncoiled. The other chain — the delayed one — is
synthesized in numerous small fragments in the opposite direction. The
two processes engage different enzymes and differ in the mechanism of
error correction. This duplicitous procedure gives rise to more errors in
the delayed chain which has made Furusawa assumed certain
evolutionary consequences involved.
A multitude of other established data and facts related to the
structural organization and functions of the genome cannot be under
estimated and ignored. These are the multigenic organization of the
genes in eukaryotic cells, which leads to the increase of their numbers
and the entire genome; the interrupted structure of the genes, as a result
of which a lot of genomes are organized as individual transcription units;
the switch on and off of introns and exons, the binding of which is
accomplished by RNA splicing; the control elemets of McClintock (1950,
1956 a, b) called mobile IS-elements (insertion sequences) capable of
breaking away and inserting themselves in new genome sites, etc. They
are believed to exercise influences on the normal activity of the genes,
the state of the genome and its stability.