function is not known (Gall, 1958; Ris, 1957). Loop formation is inter- 

 preted by Gall (1958) as a reversible physiological change which is 

 probably nongenetic. However, he points out that loops exhibit varia- 

 tions in their morphology which suggests that perhaps each loop, or 

 loop pair, represents a different genetic locus responsible for the forma- 

 tion of some particular cell product. 



Figure 4-22. Diagrammatic Representation of Lampbrush Chromosome 

 Structure. Top. enlargement of portion of single chromosome showing chro- 

 monemata and chromomere with attached lateral loops. Bottom left, a 

 bivalent with homologous chromosomes joined by two chiasmata, and 

 showing paired lateral loops. Bottom right, enlargement of section of single 

 chromosome illustrating the variation in morphology of the chromomeres 

 and loops. (From Gall, J. G., 1956. "On the Submicroscopic Structure of 

 Chromosomes," Brookhaven Syinp. Biol., 21, Fig. 7, p. 22, and Fig. 1. p. 18.) 



SOME BIOCHEMICAL CONSIDERATIONS 



As already mentioned, the nucleus contains four major chemical 

 constituents: (1) DNA, (2) RNA, (3) proteins, and (4) lipids. 



DNA 



Of the chemical constituents of the nucleus, DNA has b?en the most 

 extensively studied in terms of its molecular organization, localization, 

 and function within the nucleus. Biochemical studies have established 

 that the amount of DNA per somatic nucleus is approximately the same 



STRUCTURE AND FUNCTION OF THE NUCtEUS / 103 



