least double and that the two halves are intertwined in a relational coil 

 which becomes progressively uncoiled as the prophase advances (Figure 

 5-4). 



During this process the nuclear boundary becomes disrupted, the 

 spindle apparatus begins to form, and the nucleolus and chromocenters 

 generally disappear. 



Figure 5-4. Schematic Representation of Plectonemic and Paranemic 

 Coils Composed of Two Strands: (a) plectonemic coil made up of two 

 interlocking strands which form a relational coil when the gyres are partially 

 drawn out; (b) paranemic coil of two strands which are free to separate as 

 independent units when straightened out. (From Sparrow, A. H.. Huskins, 

 C. L., and Wilson, G. B., 1941. "Studies on the Chromosome Spiralization 

 Cycle in Trillium^ Can. J. Res., C, 19, Fig. 1. p. 325.) 



Prometaphase 



Following the breakdown or disappearance of the nuclear boundary, 

 the chromosomes tend to aggregate in a central position in the cell. In 

 plant cells, at least, this time corresponds to the first appearance of an 

 organized spindle. Wilson and Hyppio (1955) considered this position- 

 ing of the chromosomes to play an important role, both in the develop- 

 ment and normal functioning of spindle organization. Motion pictures 

 of the mitotic process (Bajer, 1957) would seem to indicate that pro- 

 metaphase collapse is characteristic of normal mitoses in plant cells. 

 Similar motion pictures such as those produced by Raymond Zirkle and 

 his colleagues indicate that the. same is true for animal cells as well. 



Metaphase 



This is the stage of alignment of chromosomes on the equatorial plate, 

 which is the midportion of the spindle apparatus. It appears to be the 

 kinetochores which are particularly attracted towards this region. They 



MITOSIS AND MEIOSIS / 123 



