320 CELL HEREDITY 



sonuil aiul nucleolar RNA can be deduced from these data. The fact 

 that tlicr(> arc breaks in both curves after five hours suggests the presence 

 of two different RNA fractions at each site, but they have not been 

 identified further. 



A much more detailed picture of the synthetic activities of chromo- 

 somes has come from autorachographic studies of particular sites in the 

 polytene chromosomes of such dipteran insects as Chironomus, Sciara, 

 and RJiyncliosciam, in which the larval chromosomes attain even greater 

 magnitudes than those of Drosopliila. Changes in morphology have 

 been observed in these chromosomes, and referred to as "puffs," and 

 "Balbiani rings.' Recently, these structural changes have come under 

 careful study in an attempt to correlate them with functional activities 

 of the chromosomes. 



Beerman has studied Chironomus chromosomes, comparing the 

 morphology of salivary-gland chromosomes with those of the polytene 

 chromosomes of the Malpighian tubules, the midgut and the rectum. 

 He reports differences in the number and location of Balbiani rings 

 and puffs in different tissues at different times in development. Puffs 

 have been followed carefully through larval development in Rhynchosciara 

 by Breuer and Pavan, who found that the process of puffing is entirely 

 reversible, although the timing varies from locus to locus. Altogether, 

 few puffs are found per chromosome compared to the number of bands. 



Recently, the uptake of radioactive precursors has been employed to 

 investigate whether macromolecular syntheses occur in relation to puff 

 formation. In a number of different species, it has already been shown 

 that DNA, RNA, and protein synthesis may occur in conjunction with 

 puffing. The three types of compounds may each be synthesized in- 

 dependently of changes in the others, or simultaneous syntheses may 

 occur. As shown by the photograph of Figure 11.7, the autoradiographs 

 of these giant chromosomes are dramatic evidences of high rates of syn- 

 thetic activity. Is puff formation a visible expression of gene action? 

 This is the working hypothesis of investigators in this field. 



To summarize the preceding section, we have seen that protein syn- 

 thesis occurs primarily in cytoplasmic RNA-protein particles, the ribo- 

 somes; that two classes of RNA molecules have been identified with the 

 process, s-RNA which carries the activated amino acids, and another 

 RNA, which is conjectured to provide a template for specific sequential 

 arrangement of the amino acids during polymerization. Protein syn- 

 thesis also occurs in the nucleus at least in some cell types. The synthesis 

 of most, if not all, RNA occurs in the nucleus; consequently, it is hard to 

 escape the conclusion that ribosomal RNA per se is synthesized in the 

 nucleus, despite the indirect nature of the evidence. An unidentified 



