Structure of human metaphase chromosomes

Abstract

Human metaphase chromosomes possesss an organized axial structure retaining the helical pattern of the chromatid macrocoil (Ohnuki-coil) after the dispersion of virtually all chromatin. This result is based on electron microscopy of chromosomes treated with alkaline pH or hot phosphate and stained with silver nitrate or phosphotungstic acid. Under these conditions a linear or spiralized axial filament has a diameter of about 90–100 nm and, consequently, a length equalling that of the chromatid macrocoil in individual chromosomes. The results are interpreted in terms of a model for packing of chromatin in metaphase chromosomes in which (1) the axial filament is composed of the interspersed interchromo-meric stretches of the uninemic chromosome fiber, (2) the axial filament is situated laterally in the chromatid macrocoil, and (3) the compaction involves (a) the initial assembly of the axial filament with the concomitant coiling of chromomeres and (b) the subsequent spiralization of the axial filament and the chromatid macrocoil.