STEREO-ELECTRON MICROSCOPY OF NUCLEAR-STRUCTURE AND REPLICATION IN CILIATED PROTOZOA (HYPOTRICHA)

  • 1 January 1981
    • journal article
    • research article
    • Vol. 25  (1) , 120-130
Abstract
Employing stereo-EM on sections (.apprx. 0.1 .mu.m thick) of the ciliated protozoa Stylonychia mytilus, Oxytricha sp. and Euplotes sp., the ultrastructure of macronuclear replication bands (RB) and micronuclei was studied. The macronuclear RB is composed of 2 zones: a forward zone showing a special chromatin organization consisting of regular 40-50 nm diameter fibers with an indication of nucleosomal substructure; and a rear zone, the site of DNA replication, consisting of a mesh of 10 nm chromatin fibers. Micronuclei exhibit chromatin strands of 60-70 nm diameter. The Bernhard staining procedure shows that the chromatin fibers of the RB and of the micronuclei remain unbleached; macronuclear condensed chromatin is bleached of stain, indicating that the replication band and the micronuclei contain chromatin in a configuration different from that of other forms of densely packed chromatin. The regularity of the chromatin fibers within the forward zone is of particular interest since it is comparable to the regularity of chromatin seen in the transcriptionally-inactive chromatin of other nuclei such as avain erythrocytes and sea urchin spermatozoa. The forward zone chromatin fibers probably consist of highly ordered arrangements of nucleosomes, associated with additional nonhistone proteins.