a relational coil. If they are virtually single, then to become relationally 

 coiled requires only that the potentially double strand twist on its own 

 axis. This theory does, however, fail to explain two occasionally re- 

 ported observations, namely, the development of a minor coil at right 

 angles to the major coil and the relational coiling of homologous chromo- 

 somes at zygotene. The former, however, is at the limit of optical reso- 



Figure 7-3. Photomicrograph of Late Diakinesis Chromosome of Trillium 

 erectum with a Schematic Representation of Four-strand Paranemic-Plec- 

 tonemic Association in a Half-Bivalent. Each strand in the diagram is a half 

 chromatid. (From Wilson, G. B., Sparrow, A. H., and Pond, V., 1959. "Sub- 

 Chromatid Rearrangements in Trillium erectum. I. Origin and Nature of 

 Configurations Induced by Ionizing Radiation," Am. J. Botany, 46, Fig. 1, 

 p. 310. Courtesy of Dr. A. H. Sparrow, Brookhaven National Laboratory.) 



lution and is considered by many to be either an optical illusion or a 

 zigzag which would be expected in a longitudinally heterogeneous thread. 

 With respect to the latter observation, there is no indisputable evidence 

 that such relational coiling actually exists, since the few cases illustrated 

 seem equally likely to be accidental overlaps. 



The electron microscope has so far added little to our knowledge of 

 chromosome geometry beyond indicating that component strands are 

 almost certainly made up of many (probably 64) microfibrils which are 



THE CHROMOSOME COILING CYCLE / 169 



