Cytogenetics

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cytogenetics

[¦sīd·ō·jə′ned·iks]
(cell and molecular biology)
The comparative study of the mechanisms and behavior of chromosomes in populations and taxa, and the effect of chromosomes on inheritance and evolution.

Cytogenetics

 

a science that deals with the regularities of heredity in relation to the structure and functions of various intracellular structures (seeHEREDITY). The principal object of study of cytogenetics is the chromosome—its morphology, structural and chemical organization, and functions and its behavior in dividing and nondividing cells (see). AS an interdisciplinary science, cytogenetics uses the methods of genetics and cytology and is closely associated with various branches of these sciences, including molecular genetics, cytochemistry, karyology, and karyosystematics. It is divided into general cytogenetics, which studies the general cellular bases of heredity, and into plant cytogenetics, animal cytogenetics, and human cytogenetics.

Cytogenetics arose early in the 20th century, after the rediscovery of Mendel’s laws in 1900; it arose in the course of the search for cytological explanations of Mendel’s concepts of segregation and independent assortment of genes (seeMENDEL’S LAWS). A great deal of information had been accumulated by this time on chromosome morphology (the Russian scientist I. D. Chistiakov, 1872, 1874; the German scientist E. Strasburger, 1875; the German scientist W. Flemming, 1882, 1892) and on chromosome behavior in mitosis and meiosis (Strasburger and Flemming; the Russian scientist P. I. Peremezhko, 1878; the Belgian scientist E. van Beneden, 1883; the German scientists T. Boveri and O. Hertwig, 1884). The presence of a paired (diploid) chromosome set was discovered in the somatic (asexual) cells and a single (haploid) set in the sex cells, and the foundations for linking chromosomes and Mendel’s “hereditary factors,” whose nature was unclear at that time, were established. In 1902 the American scientist W. Sutton and the German scientist T. Boveri, who discovered that the transmission of chromosomes from generation to generation is related to the hereditary factors (later called genes), conjectured that chromosomes are carriers of genes and are a means of assuring the continuity of traits through several generations of organisms. The main elements of the chromosome theory of heredity, substantiated and developed by the American geneticist T. H. Morgan and his school, became the theoretical foundation of cytogenetics.

In the USSR, the first to engage in cytogenetic research was S. G. Navashin. Investigating metaphase chromosomes in the giant summer hyacinth (Galtonia candicans), Navashin discovered a pair of chromosomes with a small mass, called the satellite, at one end, thereby brilliantly confirming the validity of both the theory of the individuality of chromosomes and the homology of paired chromosomes (1912). He also discovered the fundamental principle of chromosome construction from two arms, caused by the attachment of the threads of a dividing cell spindle to a strictly predetermined portion of the chromosome (1914). Two books, The Material Foundations of Heredity (1924) by the Soviet scientist G. A. Levitskii and The Cytological Basis of Heredity (1928; Russian translation, 1934) by the German scientist K. Belar, were instrumental in shaping cytogenetics as an independent science. Fundamental studies in cytogenetics were conducted by the N. K. Kol’tsov, A. A. Prokof’eva-Bel’govskaia, B. L. Astaurov, and G. D. Karpechenko.

As cytogenetics developed, the phenomena of segregation, independent assortment, genetic linkage, and crossing-over received cytological substantiation. A study of chromosome behavior in meiosis revealed that the segregation of characters in offspring is effected by the conjugation (or synapsis) of chromosomes. As a result of their migration in the first meiotic division to opposite poles of the cell, the gamete contains a single (haploid) chromosome set instead of a double (diploid) one, as found in the somatic cells of the organism. The independent assortment of genes located in nonhomologous chromosomes is due to the free recombination in meiosis of the chromosomes obtained from the father and mother. It was confirmed that genetic linkage may be broken during crossing-over as a result of the exchange of homologous portions of paired chromosomes, an exchange that leads to the recombination of hereditary material.

The cytogenetic analysis of chromosome conjugation revealed that disruption of conjugation results in incorrect disjunction of the chromosomes and in the formation of gametes with a chromosome set that is not a multiple of the haploid, that is, in aneuploidy. This leads to a decline in fertility or to sterility in plant and animal hybrids, especially remote ones. In 1927, G. D. Karpechenko devised a technique for restoring the fertility of plant hybrids, which involved doubling the chromosome set, that is, creating amphidiploid organisms. The technique is widely used in plant breeding (triticale, the wheat-rye amphidiploid, is considered very important). In 1936, B. L. Astaurov bred the first animal amphidiploid—the Asiatic silkworm (Bombyx mori). The study of chromosome conjugation, an indicator of genetic relationship, enabled the Japanese cytogeneticist H. Kihara (1924) to work out another cytogenetic method—genome analysis (seeGENOME ANALYSIS). Wheat, cotton, and other polyploid cultivated plants and their wild relatives were subjected to such analysis. The results made it possible to determine the origin of many cultivated plants, to use wild plants for breeding purposes, to enrich crops with economically useful properties, and to study the evolution of plants.

The microscopic analysis of chromosome structure and behavior in mitosis and meiosis revealed changes in the chromosome sets of plants, animals, and man (seeMITOSIS and MEIOSIS). The pioneering research of these changes, called chromosomal aberrations, was carried out on corn by the American cytogeneticist B. McClintock in the period 1929–38. The aberrations were subsequently classified, and many of their genetic consequences and the inductive effect of ionizing radiation were determined. Improved research techniques facilitated the study of the structural polymorphism of chromosomes in nature (N. P. Dubinin and coworkers; the school of T. G. Dobzhansky in the United States in the 1930’s and 1940’s). Subsequent research disclosed that many chromosomal aberrations, as well as the phenomena of monosomy (the loss of a chromosome from the set) and trisomy (the addition of a chromosome to the set), are responsible for some developmental abnormalities and many diseases in man. This stimulated the intensive development of human cytogenetics and medical genetics (seeGENETICS, MEDICAL).

The use of electron microscopy, radioisotopes, microphotometry, X-ray diffraction analysis, and other methods substantially broadened and deepened ideas regarding the ultrastructure of chromosomes (chromonema, chromatid, chromomere) and made it possible to study the chromosome material (chromatin) and the functions of chromosomes during replication and the synthesis of ribonucleic acid (transcription) and proteins (translation).

The cytogenetic method of culturing somatic cells has been widely used since the 1960’s to solve a number of genetic problems. The hypothesis of the differential activity of genes as the basis of cell differentiation was developed (the British scientist J. Gordon, 1962–76). The discovery of deoxyribonucleic acid (DNA) in chloroplasts and mitochondria (the German scientist K. Correns, 1909 and 1937, and others) stimulated research on cytoplasmic heredity and its relation to nuclear heredity (seeHEREDITY, CYTOPLASMIC).

In the 1970’s cytogeneticists focused on the study of chromosome structure and functions at the molecular level. Their research has shed light on the evolution of karyotypes and, consequently, on speciation.

Research in cytogenetics in the USSR is conducted at the Institute of Cytology of the Academy of Sciences of the USSR, the Institute of General Genetics of the Academy of Sciences of the USSR, the Institute of Cytology and Genetics of the Siberian Division of the Academy of Sciences of the USSR, the Institute of Medical Genetics of the Academy of Medical Sciences, and the Institute of Molecular Biology of the Academy of Sciences of the USSR. It is also conducted by university subdepartments of genetics and cytology. Articles are published in the Soviet journals Genetika (since 1965), Tsitologiia (1959), and Tsitologiia i genetika (1967) and in such foreign periodicals as the Canadian Journal of Genetics and Cytology (Ottawa; since 1959), Chromosoma (Berlin-Vienna; 1939), Cytogenetics (Basel; 1962), Cytologia (Tokyo; 1929), Experimental Cell Research (New York; 1950), and American Journal of Human Genetics (Baltimore, 1949).

REFERENCES

Astaurov, B. L. Tsitogenetika razvitiia tutovogo shelkopriada i ee eksperimental’nyi kontrol’. Moscow, 1968.
Swanson, C., T. Merz, and W. Young. Tsitogenetika. Moscow, 1969. (Translated from English.)
Konstantinov, A. V. Tsitogenetika. Minsk, 1971.
Tsitogenetika pshenitsy i ee gibridov. Moscow, 1971.
Karpechenko, G. D. Izbr. trudy. Moscow, 1971.
Tsitologiia i genetika meioza. Moscow, 1975.
Burnham, C. R. Discussions in Cytogenetics. Minneapolis, Minn., 1962.

V. V. KHVOSTOVA

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