The term chromosomes was introduced by Wilhelm Waldeyer
(1888). The studies showed that these are complex nucleoprotein
structures differing in shape, size and functions. Each of them displays
specific configuration with a defined quantity of DNA, which is bound to
different basic proteins (protamins and histones). Some of these protein
molecules are wound around the DNA double helix or are situated in
its grooves, and replicate simultaneously with the DNA itself.
image
Figure 2–20. Chromosome set of Crepis capilaris at the time of the metaphase plate (After M. S. Navashin; From Dubinin, 1976).
The number of chromosomes in the cells of a given species is
constant. Sexual cells normally possess a haploid chromosome set
(1n) while the somatic ones — diploid (2n). They are best observed under a
light microscope in the process of mitosis and meiosis especially at the
metaphase stage, when they are strongly spiralized and arranged in
the so-called metaphase plate. Owing to the high resolution of
electron microscopes, the replication of the chromatids has been proved in
an undisputable manner (Fig. 2–21). Human genome provokes an
understandable interest. The haploid chromosome set of the Homo sapiens
consists of 23 chromosomes and the diploid one, (called also karyotype) is
composed of 46 homologous chromosomes (Fig. 2–22).
Sex determination in the human also depends on combining the X
and Y-chromosomes. Changes in some of the chromosomes lead to
irreparable consequences — genetic and chromosome diseases, characterized
chiefly with physical abnor-malities and mental insufficiency.
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Figure 2–21. Electron micrograph of human chromosome 12 with two chromatids (After DuPraw, 1970).