Regulation of Gene Action — Molecular Basis in Higher Organisms 



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figure 40-2. Giant "lampbrush" chromo- 

 somes of the amphibian oocyte. A. Unfixed 

 chromosomes of Triturus viridescens in saline 

 solution, phase contrast, 540X- B. Semidia- 

 grammatic view of the central chromomere axis 

 with paired lateral loops. {Courtesy of J. G. 

 Gall.) 



reforms part of the main nonsynthesizing 

 chromosome axis. At the same time more 

 of the main axial thread unwinds to pro- 

 duce the thin end of the loop which pro- 

 ceeds to synthesize RNA. In any event, it 

 is clear that the morphology of a chromo- 

 somal site is closely related to its ability to 

 synthesize RNA. 



The large number of loops present in 

 lampbrush chromosomes indicates that a 

 large number of chromosomal sites are syn- 

 thesizing RNA. Although almost all the 

 granules (chromomeres) that occur along 



the main axis of lampbrush chromosomes 

 have loops, at any given time only about 2 

 per cent of the bands (comparable to chro- 

 momeres) show puffs in giant polynemic 

 chromosomes. A comparison of the pro- 

 tein, the DNA, and the RNA content of 

 liver, dipteran salivary, and lampbrush chro- 

 mosomes also suggests that the oocyte chro- 

 mosome is synthetically active at many loci. 

 It is noteworthy that the DNA content of a 

 lampbrush chromosome is about four times 

 that of a regular chromosome from a diploid 

 cell of the same newt, and that the nucleo- 



