Section 6 — Cytology 



6.6. In Vivo Studies on Mitosis in Neurospora crassa. 

 J. Weijer, A. Koopmans, and D. L. Weijer 

 (Edmonton, Canada). 



In vivo observation of karyokinesis of the 

 somatic nucleus of Neurospora crassa reveals 

 three distinct phases of nuclear division. In the 

 DNA Replication Phase the continuous genetic 

 material is oriented along the edge of a disc-like 

 nuclear structure and replicated as a ring. 

 During Division Phase the double stranded 

 filamentous nucleus divides longitudinally. 



During Chromosome Phase the divided nucleus 

 is visible as a structure consisting of seven 

 chromosomes attached to one another by 

 thread-like filaments, together with a nucleolus 

 and a centriole. 



6.7. The Communicating Pore between the Nucleus 

 and the Vacuole in Saccharomyces cerevisiae. 



Akira Yuasa (Tokyo, Japan). 



By electron-microscopy it has been confirmed 

 by several investigators that the nucleus of 

 Saccharomyces cereviseae has a definite mem- 

 brane which is composed of two-layers (Yuasa, 

 1958, 1960; Hashimoto and Gerhardt, 1960; 

 Conti and Naylor, 1960) and also that the 

 chromatin-threads exist in the nucleus (Yuasa, 

 1958, 1960; Yotsuyanagi, 1959, 1960). 



When fixed with 1 per cent aqueous solution 

 of osmic acid and stained with aceto-carmine 

 solution the pore is seen at the contacting place 

 between the nucleus and the vacuole at the 

 end of prophase by light microscopy. 



By electron-microscopy the ultrathin section of 

 the cell of Saccharomyces cereviseae also often 

 shows a figure in which the communicating pore 

 is seen between the nucleus and the vacuole. 



Lindegren (1947, 1947, etc.) already advocated 

 that the nucleus contacted with the vacuole 

 and the chromosome appeared in the latter. 

 The above-mentioned pore is thought to be the 

 communicating site of the nucleus with the 

 vacuole at the end of prophase. 



6.8. Observational and Experimental Approaches to 

 Problems of Chromosome Fine Structure. 



Helen Gay (Ann Arbor, U.S.A.). 



The basic structural component of both the 

 polytenic giant chromosome and the smaller 

 mitotic-type chromosome appears to be a fiber 

 about 100 A in diameter. According to our inter- 

 pretation, based on observational and experimen- 



tal methods, these nucleoprotein microfibrils 

 constitute a multistranded chromosome. The 

 mitotic chromosome is composed of a relatively 

 small number of cytologically separable units, 

 whereas the polytene chromosome is composed 

 of many hundreds. Treatment of living, unfixed, 

 and fixed chromosomes with low concentrations 

 of pronase, trypsin, or chymotrypsin induces a 

 separation of the chromosomal strands so that 

 the subsidiary units can be seen at both the light 

 and electronmicroscopical levels of resolution. 

 Analysis, by cytological and cytogenetical 

 methods, of the effect of deoxyribonuclease and 

 of 5-bromodeoxyuridine in producing chromo- 

 somal aberrations reveals that the chromosome 

 responds as a nucleoprotein complex, the breaks 

 in some cases extending across the whole chro- 

 mosome. This response is similar to that evoked 

 by ionizing radiations. Autoradiographic deter- 

 minations of the pattern of incorporation of 

 tritiated nucleosides and amino acids, along 

 polytene and mitotictype chromosomes supports 

 the interpretation that DNA in itself does not 

 constitute the main structural component of the 

 chromosomes in higher plants and animals. 



This research was supported in part by grants 

 GM- 10499 from the Public Health Service, and 

 GB-290 from the National Science Foundation. 



6.9. The Karyotype of the Domestic Fowl. E. H. New- 

 comer (Storrs, U.S.A.). 



There is now general agreement that the fowl 

 carries twelve well-defined chromosomes in the 

 male and eleven in the female. The female is 

 therefore heterogametic for sex with an XO type 

 of sex determination, the fifth largest chromo- 

 some being unpaired. 



In addition to these so-called macro-chromo- 

 somes, there also occurs a variable number 

 (forty to ninety) of so-called micro-chromosomes 

 which not only vary in number but range in size 

 from macro-chromosomal to the limits of mi- 

 croscopic resolution. 



Previous studies from our laboratory have 

 shown the alleged micro-chromosomes to be 

 acentric, variably heterochromatic, prone to 

 fusion and fragmentation and that they disappear 

 during the second meiotic division of spermato- 

 genesis. For these reasons they were removed 

 from the category of chromosomes and named 

 chromosomoids. 



Recent studies by an improved technique of 

 blood leucocyte culture and autoradiography 



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