Section 16 — Human Cytogenetics 



been determined provisionally. For example, 

 chromosome No. 1 appears to have 57 chromo- 

 meres and chromosomes 21 and 22 show 9 each. 



No chromocentral region has been identified, 

 even in very well spread pachytenes, but the sex 

 vesicle has been superimposed on one or more 

 chromosome pairs, giving the impression of a 

 chromocenter. 



The X and Y chromosome threads have been 

 observed inside the sex vesicle, apparently 

 attached by the tips of their short arms. The 

 long arm of the Y ends in two minute, very 

 deeply staining appendages which protrude from 

 the vesicle. The basis of the X-Y pairing is 

 obscure, but some favorable metaphases reveal 

 a knot-like structure reminiscent of a chiasma 

 at the point of association. 



At the end of pachytene or the beginning of 

 diplotene, the sex vesicle begins to disintegrate 

 and the sex chromosomes become as distinct as 

 the autosomes, each showing two chromatids. 



Pachytene cells show from one to four nucleoli, 

 none of them associated with the sex pair. Their 

 size depends upon the number of them present. 

 In most cases, they are associated sub-terminally 

 with small or medium-sized acrocentric auto- 

 somes, but there are some variations which will 

 be discussed. 



16.4. Studies on the Leucocyte Stimulating Factor in 

 Beans. Kerstin Lindahl-Kiesslins (Uppsala, 

 Sweden). 



Saline extracts from beans (Phaseolus vulgaris) 

 have been fractionated using different pro- 

 cedures. The fractions have been tested with 

 regard to their mitogenic activity, red cell 

 agglutination, serum precipitation and electro- 

 phoretic mobility. 



16.5. The Non-random Distribution of Specific 

 Chromosomes in Somatic Metaphase Figures 

 from Cultured Human Leucocytes and Their 

 Relation to the Time of Termination of Chromo- 

 some Duplication. O. J. Miller, W. R. Breg, 

 A. C. Christakos, B. B. Mukherjee and 

 A. van N. Gamble (New York, U.S.A.). 



are farther from the center of circular metaphase 

 figures, and more frequently at the periphery of 

 less selected metaphase figures than most of the 

 remaining chromosome pairs or groups. Pair 

 number 3 and group 6-12 + X share this same 

 rather peripheral location. Chromosome 1 is 

 closer to the center and less frequently at the 

 periphery of metaphase figures than any other 

 chromosome. Pair number 2, group 14-15 and 

 group 19-20 are almost as near to the center as 

 pair number 1 . 



The location of a chromosome is not related 

 to its size, but may be related to the time it 

 terminates DNA synthesis, as shown by auto- 

 radiographic studies with thymidine-H 3 . In 

 general, the chromosomes that terminate their 

 DNA replication later in the DNA synthetic 

 period than do the other chromosomes of the 

 complement tend to have a peripheral location. 



16.6. A Possible Mechanism of Dicentric Formation: 

 "Telomeric Binding" in Cultivated Human 

 Cells. P. S. Moorhead and Eero Saksela 

 (Philadelphia, U.S.A.). 



Studies of metaphase chromosome rearrange- 

 ments induced in cultured human cells by the 

 presence of the simian vacuolating agent 

 SV-40 have led to the interpretation of certain 

 abnormal chromosomes as the product of an 

 end-to-end coupling of morphologically identi- 

 fiable autosomes into a dicentric which repli- 

 cates further as an entity. Examples interpreted 

 as resulting from such "'whole-chromosome- 

 translocations" have been found within inde- 

 pendently transformed cultures, including evi- 

 dence involving diplochromosome homologues 

 following endoreduplication. Cytologic demon- 

 strations of spontaneous occurrences of this 

 process were also found in cells of a mosaic 

 strain and in the degenerative phase of long 

 term cultivated diploid strains. Interpretations 

 of the origin of various other rearrangements and 

 chromosomes losses resulting from in vitro 

 SV-40 virus infection will also be considered in 

 the context of findings from studies employing 

 human diploid strains and current attempts to 

 demonstrate similar effects with human viruses 

 adeno-12 and adeno-18. 



The chromosomes are not distributed at 

 random in flattened metaphase figures from 

 colchicine, hypotonic sodium citrate treated 

 human leucocyte cultures. The Y chromosome, 

 one of the X chromosomes, and pair number 13 

 tend to occupy a peripheral location. Pair 

 number 21 and the chromosomes in group 17-18 



16.7. Determination of the DNA Content of Cell 

 Nuclei and Individual Human Chromosomes by 

 Photographic Colorimetry. E. M. den Tonke- 

 laar, J. L. G. Gaillard, P. van Duun and 

 A. Schaberg (Leyden, the Netherlands). 



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