Section 6 — Cytology 



variation of RNA synthesis is a characteristic 

 feature of polytene cells of Chironomus. Attempts 

 to control the rhythm of synthesis in salivary 

 glands by modifying light and temperature con- 

 ditions have so far failed. 



6.25. Localization of Deoxyribonucleic Acid exclu- 

 sively in the Bands of Drosophila Salivary Chro- 

 mosomes. Dale M. Steffensen (Urbana, 

 U.S.A.). 



Drosophila salivary chromosomes were labeled 

 with HMhymidine to study the distribution of 

 radioactive DNA by quantitative autoradio- 

 graphy. Stretched salivary chromosomes were 

 examined for the distribution of silver grains 

 (radioactivity) produced from tritium over bands 

 and interbands. The evidence leaves little doubt 

 that DNA is primarily if not exclusively in the 

 bands. On the other hand the interbands would 

 appear to contain no more radioactivity than 

 background and therefore little or no DNA. 



A variety of physical considerations were ex- 

 amined and it was calculated that DNA must be 

 only in the bands and probably absent in inter- 

 bands. The theoretical evaluation showed that 

 radioactivity would have been detectable if 

 DNA were in interbands. The presence of DNA 

 in interbands proved to be highly unlikely. All 

 in all it would appear that Bridges was right all 

 along in his hypothesis that genes (DNA) are 

 located only in the bands. 



As might be expected there was intense radio- 

 activity from heterochromatic regions indicating 

 a high concentration of DNA. Regions with few 

 bands and "puffs" exhibited little radioactivity. 

 In some puffs, DNA may be absent altogether. 

 The temporal inactivity of "heterochromatin" is 

 discussed in light of recent evidence about 

 localized RNA synthesis and chromosome differ- 

 entation. 



This study was supported by the National Sci- 

 ence Foundation (NSF G 17600). 



6.26. Structure and Function of the Y-heterochromatin 

 in Drosophila. G. F. Meyer and O. Hess (Tu- 

 bingen, Germany). 



In spermatocyte nuclei of Drosophila structural 

 modifications of the Y-chromosome occur which 

 have been shown to be homologous to the loops 

 of lampbrush chromosomes. Several pairs of 

 such structures can be distinguished on a mor- 

 phological basis (e.g. in D. hydei, "threads", 



"clubs", "pseudonucleolus" and "tubuli"). By 

 analogy with lampbrush loops these structures 

 have been considered as regions of the Y-heter- 

 ochromatin whose activity is increased enorm- 

 ously. This can now be directly demonstrated by 

 autoradiography and by experiments using 

 actinomycin C. Evidence for the genetic func- 

 tions of loop-forming loci comes from a study 

 of the correlation between the involved regions 

 of the Y-chromosome and the length of sperma- 

 tozoa. Spermatozoal length varies between and 

 within species according to the total mass, the 

 degree of differentiation, and the number of the 

 individual types of Y-chromosomal loops. It 

 seems that almost any alteration in the quantity 

 of Y-heterochromatin results in changes of the 

 sperm length. In a X-ray induced Y-autosome 

 translocation in D. hydei the two translocated 

 fragments of the Y-chromosome were found to 

 carry two loop-forming loci each. In the trans- 

 location stock males occur which are deficient 

 for one or the other Y fragment and which, 

 correspondingly, lack two of the four loop pairs. 

 These Y-deficient males produce immotile sperm 

 which are shorter than normal sperm. Males 

 with partial or complete duplications of loops 

 are also found and show the expected increase in 

 sperm length. The chromosomal modifications 

 described here are unique in that they are formed 

 only by the (heterochromatic) Y-chromosome. 



6.27. Amounts of DNA and RNA in the Chromosomal 

 and Nucleolar Regions of Isolated Plant Nuclei. 



John McLeish (Bayfordbury, Great Britain). 



Amounts of both DNA and RNA were deter- 

 mined in nuclei isolated from the root apices of 

 different species of higher plants. A new method 

 of isolation was used W by which approximately 

 50 per cent of the nuclei known to be present in 

 the roots could be recovered. There was no 

 significant loss of nucleic acids from these nuclei 

 during the isolation procedure. The plants chosen 

 included both closely and distantly related spe- 

 cies; a polyploid series within one genus; and the 

 diploid and tetraploid forms of a single species. 

 The results demonstrated the absence of any 

 correlation between the amounts of DNA and 

 total nuclear RNA; the high DNA: RNA ratios 

 found in most species suggested that, at any given 

 time, a lot of the DNA is not associated with 

 RNA. 



A new technique for the isolation of nucleoli 

 from these nuclei has now made it possible to 

 estimate the amounts of DNA and RNA in both 

 the chromosomal and nucleolar regions. Al- 

 though it can be stated that there is no correlation 



109 



