presumably in part DNA helices. So far as we are aware the only inter- 

 pretation of electron micrographs which attempts to tie together the 

 submicroscopic picture with that revealed by the light microscope is the 



mm Of 2 «tcw(88/tuf 

 3W SOMATIC mAPHKSi cmomum 



S^ratlon i$ possflMe in s^h « 



3 oniers * coHing. 



•W»l Argenrw Nstiowl l*. i^jg 



Figure 7-5. Model of Two Microfibrillae Showing Three Levels of Coiling 

 in a Somatic Metaphase Chromosome. (From Nebel, B. R., 1960. "On the 

 Structure of Mammalian Chromosomes During Spermatogenesis and After 

 Radiation, with Special Reference to Cores," in "Fourth Intern. Conference 

 on Electron Microscopy." Springer-Verlag, Fig. 1, p. 228. Courtesy of 

 Dr. B. R. Nebel, Argonne National Laboratory.) 



one offered by Nebel (1959-60). Essentially his model consists of a 

 hierarchy of three levels of coiling, including the minor coil (Figure 7-5). 

 The question of the reality of the minor coil, however, is still open, with 



THE CHROMOSOME COILING CYCLE / 171 



