Contributions of Electron Microscopic Spreading Preparations (“Miller Spreads”) to the Analysis of Chromosome Structure

Abstract

Before introduction of the chromatin spreading technique by Miller and Beatty in 1969 (Miller and Beatty 1969 a-d), relatively limited information was available concerning the submicroscopic organization of chromosomes in mitosis and interphase. Thus, at the end of the 1960s the general picture that emerged from inspection of ultrathin sections of cell nuclei, isolated chromatin, and of whole mount preparations was that mitotic chromosomes and interphase chromatin consisted of a complex meshwork of irregularly sized and knobby fibers with diameters ranging from 20 to 30 nm (for reviews see Ris 1969; DuPraw 1970; Solari 1974; Ris and Korenberg 1979; Hozier 1979). Although the internal organization of chromatin fibers was particularly suitable for study by surface spreading on an air-water interface, originally introduced by Gall (1963), neither a discrete subunit organization was recognized nor could the “thick” chromatin fibers be reproducibly unfolded into thinner fibers which were believed to be transcriptionally active (Gall 1966). Hence, electron microscopic methods were unavailable to study features of chromosomes during interphase, i.e., their functional subdivision into transcriptionally active and inactive domains.