Section 6 — Cytology 



6.19. The Variable Characters of Chromosomes. 



Atif Sengun (Istanbul, Turkey). 



The chromosomes carry the hereditary units, 

 the genes, and are responsible for the continuity 

 of life and of the characters of an organism. 

 Therefore, it has been accepted for many years 

 that the chromosomes should have a static 

 organization represented by a thread with a 

 definite number of chromomeres, varying in 

 size and which occupy a special place on the 

 chromonema. But a comparison of the homolo- 

 gous chromosomes shows a variation of chromo- 

 some morphology, e.g. in corresponding phases 

 of mitotic and meiotic divisions or in different 

 tissues of the same animal, etc. The morphologi- 

 cal or structural differences of the homologous 

 chromosomes described in relevant literature 

 are: (a) the degree of spiralization, (b) the 

 length and (c) the thickness (the diameter) of 

 the chromosome, (d) the number and the 

 localization of differentiated, specific, regions 

 such as the heterochromatic parts, the puffs and 

 the macrochromomeres, etc., (e) the amount of 

 the non-hereditary substances including the 

 metabolic DNA and RNA. All these variable 

 characters together form the investigated and 

 visible chromosome. 



A survey of the literature reveals: (1) the 

 above-mentioned characters are controlled by 

 internal and external factors — including the 

 microenviromental ones, (2) an alteration of a 

 character of the chromosome is correlated with 

 certain, even with definite physiological variation 

 of chromosome, (3) Some substances are formed 

 and given off into the surrounding medium by 

 the chromosome, (4) certain structures of the 

 chromosomes are responsible for the formation 

 of these non-genetical, temporary substances. 

 Therefore, it may be suggested that the visible 

 structure or the morphology of the chromosomes 

 represent the phenotype of a chromosome which 

 is merely a product of non-visible internal and 

 external factors. According to the above- 

 mentioned conclusion the question "What is the 

 genotype of a chromosome?" is still obscure. 



6.20. Heterocyclicity of the System of Cell Nucleus. 



A. Prokofieva Belgovskaya (Moscow, 

 U.S.S.R.). 



The study of the behaviour of the parent 

 chromosomes of the nuclei in Drosophila 

 melanogaster salivary gland cells has shown 

 that in a number of cases lack of conjugation of 

 homologous regions is caused by a difference 

 of the cyclic state of the chromosomes or their 



corresponding regions. Heterocyclicity of chro- 

 mosome regions becomes increased in the cases 

 of rearrangements. Distal regions IABI brought 

 near to heterochromatic region 20A in the 

 X-chromosome of homozygous female sc 8 differ 

 by their states and do not conjugate in 27 per 

 cent of the nuclei. 



The study of the cell nucleus on early stages of 

 development of Salmo salar, Corregonus baeri 

 and Cyclops serrulatus showed that parent 

 chromosomes till to the stage of middle gastrula 

 remain isolated and up to the late blastula differ 

 from each other in many respects, such as: 

 character of spiralization of prophase chromo- 

 somes, time of their mitotic reorganizations and 

 sometimes the intensity of the nucleal reaction. 

 During prophase the parent chromosomes form 

 two separate skeins of threads oriented towards 

 opposite cell poles. Parent metaphase chromo- 

 somes in many blastomeres are oriented in 

 different plates. In late blastulae parent chromo- 

 somes distinctly differ in their ability to form the 

 material of nucleolus. 



While investigating chromosomes of ir- 

 radiated embryos of Salmo salar and Misgurnus 

 fossilis (X-ray, 800r, stage of 8 blastomeres) 

 differential damage of parent chromosome 

 complexes could be stated. This could be 

 connected with a difference of their cyclic states 

 at the time of irradiation. 



Heterocyclicity of parent chromosome sets is 

 clearly expressed in a primary culture of human 

 embryonic fibroblasts. It leads to the formation 

 in a number of cells at various stages of mitosis 

 of separate chromosome complexes. With the 

 aid of thymidin - H 3 and autoradiography it 

 could be shown that homologous parent chro- 

 mosomes differ in relation to the reduplication 

 cycle of their DNA as well. This peculiarity is 

 highly characteristic for the homologues of the 

 X-chromosome pair, and slightly expressed for 

 some autosomes in the primary culture of 

 embryonic fibroblasts and leucocyte cultures of 

 periferal blood. So far as observations show 

 the high grade of heterocyclicity of parent 

 chromosomes present during early stages drops 

 down in the succession of cell generations. It is 

 possible that the levelling of parent chromosome- 

 cycles presents one of the factors of chromosome 

 conjugation and transformation of mitosis into 

 meiosis. 



The data obtained confirm the assumption of 

 former cytologists concerning prolonged inde- 

 pendency of parent chromosome sets (Hacker, 

 1895; Amma, 1911). These data are considered 

 in the light of the hypothesis offered by the 

 author in 1948, according to which hetero- 

 cyclicity of the system of the cell nucleus is 

 evolutionary fixed by the peculiarities of the 



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