Ultrastructure of metaphase chromosomes and interphase nuclei

Abstract

Electron microscopy of whole-mount preparations of chromosomes and nuclei was greatly improved upon over earlier studies, when techniques were developed to expand the specimen and to preserve the expanded state during drying. When the earlier procedures were followed, the thick and dense chromosomes and nuclei always appeared nearly opaque to the electron beam and yielded little information regarding internal structure. With the newer techniques, it has been possible to study structure in detail deep within these specimens. The chromosomes of the plant Vicia faba are composed of a very long thin (5-20 nm) fiber which consists of about 38% DNA, with a presumed equal amount of histones; and a thicker 15-55 nm net-like component consisting of the remaining 24%, as nonhistones. Our interests concern determining the structural relationship of these components in chromosomes and nuclei of root-tip cells of this same plant.Fig. 1 shows a Vicia faba metaphase chromosome expanded with EDTA and processed by freeze-substitution. Inset, at the same magnification, shows an untreated similarly processed control chromosome.