Surface structure of isolated metaphase chromosomes

The relationship between the surface protuberances of mitotic chromosomes isolated from HeLa cells and the underlying fiber organization was investigated by scanning electron microscopy (SEM). Chromosomes were isolated in the presence of 5.0 mM Mg²⁺ by a method which avoids the use of organic solvents and extremes of pH. Chromosomes in 5.0 mM Mg²⁺ are highly condensed with a relatively smooth surface structure. In 1.5 mM Mg²⁺, a knobby surface substructure became apparent, with the protuberances having a mean diameter of 691 ± 96 Å. The diameter was 647 ± 76 Å at a magnesium concentration of 0.5 mM, but was only 349 ± 52 Å at a concentration of 0.15 mM. In 0.05 mM Mg²⁺, the mean diameter had decreased to 299 ± 47 Å and the chromosomes had expanded such that the underlying fibers had become a predominant feature of the micrographs. The knobby appearance of the chromosomes most likely reflects a radial arrangement of the fibers, which loop back at the peripheries of the chromosomes.