The Variable Condition of Euchromatin and Heterochromatin

Abstract

Publisher Summary This chapter discusses the variable condition of euchromatin and heterochromatin. Several terms were used to describe late-replicating chromosome material, replacing the customary term heterochromatin which was introduced by Heitz. The varying degree of condensation of meiotic chromosomes was called heteropycnosis. The chapter discusses the structure characterization of late-replicating chromosome material. According to the electron microscope studies euchromatin and heterochromatin both consist of 200-A fibrils but differ either in manner of spiralization or in condensation system. A varying content of nucleic acid and a different degree of spiralization are commonly held responsible for the different stainability. Heterochromatin in the interphase cell nucleus can fuse into one body (the collective chromocenter) or can be distributed in several nuclear regions. The Lyon hypothesis explained several puzzling phenomena and was of great help in the development of human genetics in spite of its deficiencies. Inactivation, however, represents only one aspect of the mode of action of late-replicating chromosome material. The cytological variability and the many clinical symptoms related to aberrations of late-replicating autosomes show clearly the great significance attached to the mode of action of this chromosome material.