292 Raphael Falk
According to its most widely accepted modern connotation, the word
“gene” designates a DNA molecule whose specific self-replicating struc-
ture... become[s] translated into the specific structure of a polypep-
tide chain.
This concept of the “structural gene” accounts for the multiplicity,
specificity and genetic stability of protein structures, and it implies
that such structures are not controlled by environmental conditions or
agents. [Jacob and Monod, 1961, 318]However, the model indicated not only the existence of a new class of genes – or
were these really genes? — regulatory genes, but also the existence of polygenic
units of transcription: the genes forβ-galactosidase, for permease, and for acety-
lase were all transcribed in a regulated manner into one polycistronic messenger-
RNA. To the extent that these genes were meaningful discrete structural entities,
discreteness occurred only at the stage of translation of the nucleotide sequences
into amino-acid sequences on the ribosomes. In other words, genes were units of
the translation complex rather than ones of information encoded in the DNA.
Thein vitro hybridization of separated DNA strands or of DNA and RNA
strands opened new vistas for genetic analysis. Experiments of hybridization of
RNA from various tissues of the same organism with its DNA established the no-
tion that tissue differentiation is accompanied by differential activation of genes,
and thus — together with the discovery of genetic regulatory systems – allowed a
fresh approach to the analysis of molecular control of differentiation. The possi-
bility to hybridize strands of DNA moleculesin vitroalso opened the path to the
research of evolution to overcome the barrier of inter-specific hybridization at the
molecular level [Hoyeret al., 1964].
The Pandora Box was, however, opened wide in 1968 when Britten and Kohne
measured the rate at which the sheared DNA of an organism that was separated
into single strands, reassociated to form double strands. The rate of the reasso-
ciation or reannealing of the DNA strands is a function of the complexity of the
DNA, but much of the DNA of eukaryotes was found to reanneal considerably
faster than expected. This had only one explanation, namely, that long sections
of the that DNA were redundant and highly repetitive [Britten and Kohne, 1968].
In situ hybridization of radioactively labeled DNA revealed that in the mouse the
most highly repetitive sequences (over a million repeats) were concentrated in the
heterochromatic segments of the chromosomes, adjacent to the centromeres [Par-
due and Gall, 1970]. These are chromosome regions of low genetic informational
content, apparently involved in securing regular segregation of chromosomes in the
eukaryotic cell division. However, many more repetitive sequences were found to
be intercalated in the “genetically active” euchromatic DNA segments. It was sug-
gested that these were parasitic, “selfish DNA” segments [Doolittle and Sapienza,
1980; Orgel and Crick, 1980]. Numerous transposons, or transposable elements
— DNA sequences able to insert at many locations in the genome, without se-
quence relationship with the target locus — were indeed found to be present by