definite number of chromosomes serving the purpose of reproduction. The
term genome was introduced by the German biologist Hans Winkler in 1920
for denoting the totality of all genes in a haploid chromosome set. At present
the term genome encompasses DNA as well as RNA, when it is the major
hereditary material, plus the extranuclear DNA-molecules located in certain
cell organelles autonomously replicating.
Figure 2–16. Scheme of crossing-over (Adapted after Janssens, 1909).
In prokaryotic organisms (bacteria and blue-green algae) which do
not possess a well-defined nucleus with a nuclear membrane, the
genetic material represents a circular DNA-molecule forming the so-called
nucleoid. In bacterial cells for example, one, two or more
nucleoids are observed (Fig. 2–17 A, B) whose number varies depending
on their development stage and the cultivation conditions. In shape,
even in the same cell, they are very different — rounded, stick-like, V-shaped,
T-shaped, U-shaped, etc. Under the phase-contrast microscope
in non-stained living cells they look more diffuse rather than in the
processed stained preparations.