316 



CHAPTER 34 



SEGMENT 



CHROMOSOME B 



PRE-PUFF 



PUFF 



POST- PUFF 



50 



FIGURE 34-8. Puffing and iinpnffing in a region of a salivary gland chromosome of 

 Rhyncosciara. {Courtesy of G. Rudkin.) 



behaves the same way in this and in other 

 respects. It can be inferred (from page 19) 

 that the synthesis of DNA occurs during the 

 metabohc stage. Not only does the synthesis 

 sometimes require a period of several hours 

 for completion (so that some genes replicate, 

 or are replicated, before others), but it has 

 been shown that the DNA in heterochromatin 

 (p. 181) is synthesized at a different time 

 (sometimes later) than DNA in euchromatin 

 (p. 181), even when the synthesis of both is 

 traced in the same chromosome. This has 

 been found both in animal and plant organ- 

 isms. 



Besides different portions of chromosomal 

 DNA being synthesized at different times, 

 there is also evidence that DNA is synthesized 

 in disproportionate amounts by certain re- 

 gions of polytene chromosomes. In the 

 salivary gland cells of Rhyncosciara larvae, 

 there is, for example, a disproportionate in- 



crease in the manufacture of DNA in regions 

 of the polytene chromosomes that tempo- 

 rarily puff out (Figure 34-8). Different chro- 

 mosome regions puff and unpuff at specific 

 times in the life history of the salivary gland 

 cell. Moreover, in the polytene chromosomes 

 found in other tissues of this larva, a given 

 chromosome region will puff and unpuff at 

 different stages of larval development. 



Such results suggest that the disproportion- 

 ate synthesis of DNA is concerned with 

 developmental changes and differentiation. 

 That such disproportionately synthesized 

 DNA, or other DNA, may leave the nucleus 

 to produce an effect in the cytoplasm is sup- 

 ported by various evidences. In a number of 

 organisms (for example, the fungus gnat, 

 Sciara), some chromosomes are regularly 

 eliminated from the nuclei of cells destined 

 to differentiate in particular ways; the possi- 

 ble extrusion of Feulgen-Rossenbeck staining 



