to move poleward of metaphase chromosomes which have failed to 

 undergo kinetochore cleavage in the presence of iodoacetic acid (Wilson 

 and Morrison, 1958) suggest that the chromosomes themselves possess 

 some capacity for autonomous movement. No matter what kind of 

 model one proposes to explain chromosome movement, the assumption 

 cannot be reasonably entertained that the chromosomes are merely inert 

 bodies which are pulled around by the spindle fibers. In the last analysis, 

 one must look at the postprophase part of active mitosis as a changing 

 equilibrium between the chromosomes and spindle. 



SYNAPSIS AND CHIASMA FORMATION 



Synapsis 



Any satisfactory theory of mitotic mechanisms must be able to explain 

 not only the ordinary and extraordinary movements of chromosomes 

 but also pairing, synapsis, and resultant chiasma formation. To date 

 there is no reasonable explanation of these phenomena. Darlington 

 (1937) has advanced the theory that a chromosome must necessarily 

 exist in a double condition, and that pairing of homologous chromo- 

 somes is an attempt to satisfy this requirement at a stage when each 

 individual chromosome is single. Aside from the fact that this theory 

 is dependent on the singleness of the leptotene chromosome, which is 

 by no means generally accepted, it dodges the issue of the actual force 

 involved. It has been assumed generally that the force of attraction is 

 electrostatic in nature, but even a casual analysis of the conditions is 

 sufficient to show that this idea leads to very great complexity. The 

 authors' opinion is that the force of synapsis is more likely to be truly 

 chemical, and may depend partly on the special arrangement of the 

 chromosomes and their degree of duplication. 



Chiasma Formation 

 Even if we could solve the question of synapsis, there remains the 

 question of chiasma formation. No really satisfactory hypothesis has yet 

 been advanced. Those mentioned below have gained some support, and 

 some have been widely accepted, but all fail to satisfy some require- 

 ments. 



Torsion Theory 



Chiasma formation may be caused by torsion resulting in strain break- 

 age as a result of internal twisting and entwining of homologues (Figure 

 6-9 (a) and (b)). This idea, proposed by Darlington (1937), depends 



152 / CHAPTER 6 



