The metaphase chromosome ultrastructure. II. Helical organization of the basic chromosome fiber as revealed by acute angle metal deposition.

The acute angle metal depositon technique, discloses a helical chromosome fiber substructure. The microdensitometric analysis of the fiber parameters shows that a fibril 152 A in diameter coils to form the basic chromosome fiber 295 A in diameter with a pitch of 247 A. These figures should be corrected because of the thickening due to metal deposition, the real dimensions would be closer to 112 A for the fibril diameter and 255 A for fiber diameter. The real pitch value is probably somewhat smaller than 247 A (approximatively 200 A) as the values were obtained by analysing slightly stretched fibers. These results support the helical model of the chromosomes fiber organization and show that, most probably, the metaphase chromosome fiber as seen in electron microscopy is the second order of helical packing.