denaturation, cleavage of such linkages may possibly be enzymatic in 

 character. The finding that glycolytic inhibitors (i.e., iodoacetic acid, 

 iodoacetamide) interfere with cleavage of the kinetochore and, conse- 

 quently, prevent anaphase movement of the chromosomes (Wilson and 

 Morrison, 1958) suggests that glycolytic activity may be necessary for 

 kinetochore cleavage and that such cleavage may be related in some way 

 to initiation of enzymatic activity necessary for the splitting of the 

 — SS — bonds of the spindle protein. This cleavage would lead to the 

 release of — SH groups as the result of disruption of intermolecular 

 — SS — bonding. Following release of — SH groups, there is then the 

 possibility that folding of the spindle protein molecules occurs, with 

 the subsequent formation of intramolecular — SS — linkages. Disruption 

 of secondary bonding, if such exists, would be expected to occur also 

 as part of the total process involved in spindle breakdown. This se- 

 quence of events would result in a progressive folding of the spindle 

 protein and would tend to shorten the spindle. 



The conversion of — SH groups to intramolecular — SS — linkages 

 postulated as occurring during anaphase may be an inherent property 

 of the spindle protein and independent of any redox mechanism. This 

 last point is quite compatible with the lack of effect of oxidants and 

 respiratory inhibitors (cyanide, malonic acid, urethane) on cells under- 

 going active mitosis, particularly those cells in postprophase stages of 

 the mitotic process. The folding process which accompanies the shift of 

 — SH to intramolecular — SS — linkages following metaphase could pro- 

 vide a possible mechanism whereby anaphase movement of chromo- 

 somes is accomplished. The decrease in birefrigence of the chromosomal 

 fibers which extends ahead of the moving chromosomes during anaphase 

 (Inoue, 1953; Swann, 1951) suggests that the loss of molecular orienta- 

 tion of the spindle protein is a progressive event and is initiated in the 

 region of the chromosomes. Recent cytochemical studies of Kawamura 

 and Dan (1958) have shown that protein-bound — SH groups are de- 

 tectable during telophase in only those parts of the spindle immediately 

 adjacent to the chromosomes. This observation is consistent with the 

 idea that kinetochore cleavage and spindle dissolution following meta- 

 phase is associated with the cleavage of — SS — linkages and the release 

 of — SH groups. 



The vital question of the relation of the spindle to alignment of the 

 chromosomes on the metaphase plate and the subsequent movement of 

 the chromosomes into anaphase, has never been answered satisfactorily. 

 One obvious possibility is that there exists an actual attachment of the 

 spindle fibers to part of the chromosomes, namely, the kinetochore, to 



150 / CHAPTER 6 



