Some observations on chromatin fibers of isolated pea nuclei

Abstract

Nuclei were isolated from apical portions of 5- to 7-day-old pea seedlings, and chromatin fibers were studied in sections of intact nuclei after the sections were fixed in glutaraldehyde followed by OsO4. For study of whole mounts, isolated nuclei were ruptured by two methods: repeated freezing and thawing; washing nuclei with distilled water on the grids. Whole mounts of ruptured nuclei were negatively stained with phosphotungstic acid. Chromatin fibers prepared in this way had diameters of 35 Å to 100 Å, while the diameters of the fibers in thin sections fell in the range of 100 Å – 140 Å.In some distilled-water-ruptured preparations, the chromatin fibers resembled a beaded string, and the material composing the beads was extractable with 2 M NaCl. As a result of salt extraction, a fine fibrillar core became evident and was stained in uranyl acetate. The diameter of the core was about 20 Å – 35 Å.Variations in the diameter and granular appearance of negatively stained fibers seemed to be primarily caused by uneven distribution and local accumulation of the "sheath" material. A chromatin fiber appears to be constituted of a single central deoxyribonucleic acid (DNA) core of about 20 Å to 35 Å, surrounded by sheath material.