7 



The Chromosome 

 CoiUng Cycle 



A SOMATIC metaphase chromosome consists of two more or less en- 

 twined chromatids, each containing a chromonema (which is probably 

 at least double) in the form of a helical coil (Figure 7-1). The nature 

 and origin of this coil has been the subject of very considerable study 

 and speculation. Most of the more critical studies have been based on 

 the analogous coil of first metaphase and first anaphase of meiosis, 

 especially in plants with large chromosomes. The equivalent meiotic 

 coil (Figure 7-2) is longer-gyred, looser, and more amenable to detailed 

 study. At first anaphase the chromosome consists of two chromatids, 

 relatively widely separated except at the kinetochore. With some mate- 

 rials and by special methods, preparations can be made to reveal the 

 details of coil structure and allow accurate studies of such things as the 

 number of gyres, size, and direction of coiling. 



From about 1926 to 1940 a large number of papers were published 

 on this subject. The most active workers were Darlington and his associ- 

 ates (England), Nebel (Geneva, N.Y.), Huskins and his associates 

 (Canada), and Kuwada and his students (Japan). These workers fall 

 fairly neatly into two groups on the question of the origin of this coil. 

 Nebel (1939) refers to them as the torsional and matrical schools. The 

 basic idea of the former group (Darlington, Kuwada, and Nebel) visu- 

 alizes the coil as being the more or less direct result of the torsion set up 

 by a hypothetical molecular coil. A reasonably adequate model is pro- 

 vided by a twisted elastic band, which tends to relieve the torsion by 



164 



