its component parts (Mazia, 1955, 1957). Mazia (1955) found that 

 the spindle fibers of the isolated mitotic apparatus could be put into 

 solution by reducing agents such as alkaline thioglycollate. On the basis 

 of this observation, he concluded that the spindle fibers were composed 

 essentially of small protein molecules associated end to end by — SS — 

 (disulfide) linkages to form elongate protein chains. Solution of the 

 mitotic apparatus could also be accomplished by the use of urea, which, 

 however, produced a viscous solution. Mazia interpreted this finding 

 as indicating that secondary bonds (presumably hydrogen bonds) had 

 been disrupted and that the — SS — linked protein chains or fibrils of 

 the spindle gel were probably condensed laterally by means of secondary 

 linkages. This could produce oriented clusters of protein chains and so 

 account for the microscopically visible fibers of the spindle. The achro- 

 matic component of the mitotic apparatus (asters, spindle), isolated 

 from colchicine-treated sea urchin eggs, is also found in the form of a 

 structureless gel. As pointed out by Brachet (1957), this observation 

 suggests that the protein material which normally constitutes the spindle 

 is present in these eggs, but that the typical fibrillar organization to form 

 the functional spindle is prevented by colchicine. The marked loss of 

 birefrigence of spindle material reported to occur in certain animal cells 

 after colchicine treatment (Inoue, 1952; Swann and Mitchison, 1953) 

 supports this view. Mazia (1955) concluded that colchicine might pro- 

 duce its effect by interfering with the establishment of secondary bonding 

 which he regarded as responsible for the orientation and geometry of 

 the fully formed spindle. Biochemical studies on the isolated mitotic 

 apparatus indicate that the spindle and asters consist of simple proteins 

 of characteristic composition (Mazia, 1955, 1957). It has also been 

 demonstrated that at stages where no organized spindle is evident there 

 are present proteins with characteristics similar to those making up the 

 functional spindle (Mazia, 1957). In summary, the studies on the 

 isolated mitotic apparatus have demonstrated that ( 1 ) the spindle fibers 

 are real structures, (2) the spindle protein has a characteristic compo- 

 sition, (3) the linear orientation of the spindle fiber is probably main- 

 tained by means of — SS — linkages, and (4) the particular kind of 

 protein from which the spindle is organized is present at prespindle 

 stages. 



The idea that an oxidation-reduction cycle might be associated with 

 formation of the mitotic spindle was first proposed by Rapkine (1931) 

 on the basis of studies dealing with variations in the concentration of 

 soluble — SH (sulfhydryl) during cleavage of sea urchin eggs. Rapkine 

 found that the level of soluble — SH decreased during interphase and 



146 / CHAPTER 6 



