Genetic Analysis 263deficiencies of up to several dozen “bands” of the polytenic chromosomes, allowed
the assignment of one “band”, 3C2, common to all deletions, as the site of the
genewhite[Demerec and Hoover, 1936].
Prokaryotes have no nucleus defined by a membrane, and no chromosomes that
undergo concerted mitotic (or meiotic) segregation as do eukaryotic chromosomes
at nuclear division. Notwithstanding, genetic analysis revealed the existence of
chromosomal structures also for bacteria as well as viruses (for a time, they were
called “genophores”). Processes in bacteria, similar to sexual mating or analo-
gous to them, such as transmission of DNA fragments by vectors such as viruses
or plasmids (transduction) or transfer of naked DNA fragments (transformation),
or, in viruses, by mixed multiple virus infection of the same host cell, produced
merozygotes — partial hybrids, or incomplete zygotes — which proved amenable to
detailed genetic analysis (see the following sections). The genetically constructed
chromosome map of bacteria was eventually visualized “cytologically” by radioac-
tive labeling the multiplying chromosome [Cairns, 1963].
3.1 Genetic analysis of the molecular chromosome
The chromosomal theory of inheritance assigned genetic meaning to the cellular
structures observed by cytologists. Although the chromosomes were microscopi-
cally observable only during cell division, cytologists have produced evidence that
these were the highly condensed structures of permanent entities as expected by
genetic analysis. With the advancement of molecular analysis the chromosomes
were viewed as highly organized molecules of DNA, to which proteins are attached.
Only toward the end of the twentieth century attention was directed at the roles of
proteins not only in the organization of the chromosomes but also in their function.
Once molecular analysis of the chromosome became available molecular-signposts
became important genetic-markers along the chromosomes. A long series of re-
striction enzymes each of which cuts the DNA strand in a specific pattern and at a
specific sequence of 3-6 nucleotides provided the first DNA-level chromosome maps
of Restriction-Fragments-Length-Polymorphism (RFLP). With the advancement
of whole genome DNA sequences, single nucleotide polymorphisms (SNPs) as well
as variability in the blocks of short repetitive DNA-sequences (micro-satellites)
polymorphisms, became a most efficient mapping tool of genetic engineering.
Cytogeneticists looked for the functional genetic correlates of the chromosomes’
morphological structures. Thus, the centromeres were identified the sites of the
spindle-fiber-attachment at cell division, and the heterochromatically stained re-
gions were recognized as regions of poor genetic contents, respectively. On the basis
of genetic analysis considerations Muller defined another organelle at the tips of
the chromosomes, the telomeres, that were specific monopolar end-structures, in
difference from all other chromosome site, which were bipolar and “sticky” when
broken [Muller, 1940; Muller and Herskowitz, 1954]. Repeated attempts were made
to correlate the “banding” of the polytenic chromosomes with (individual) genes
and the inter-band with inter-genes, as was later tried with the bands and inter-