III. THE EFFECTS OF AMINES ON THE STRUCTURE OF ISOLATED NUCLEI N. G. ANDERSON and CAROLYN B. NORRIS

THE volume of many interphase nuclei and of condensed chromosomes has been considered to reflect the degree of condensation of a nucleoprotein gelwork [9]. Factors that may produce volume changes in isolated nuclei have therefore been explored in the hope of finding mechanisms that may be active in chromosomal condensation during cell division. At least three factors control nuclear volume in the cell. For a given somatic cell type, the volume of the nucleus appears to be a reflection of the amount of deoxyribonucleoprotein (DNP) present. Thus liver cell nuclei fall into discrete volume classes [lo-12, 19, 23, 29, 33, 401 whose volumes are proportional to the ploidy of the nuclei and to their DNA content [ 1, 2, 12, 141. The second factor is a tissue-specific one illustrated by the large differences in nuclear volume seen in different somatic tissues having the same DNA content, in oocytes, and in sperm heads. The third factor is the ionic environment of the nucleus. The isolated rat liver cell nucleus swells and shrinks in response to changes in pH, ionic strength, and ionic composition in a manner that parallels the volume changes (or coiling) of DNA in solution [lo]. Thus liver cell nuclei and cerebral cortex nuclei swell in distilled water, shrink in very dilute calcium chloride or in acid, but still maintain their tissue-specific volume differences [9]. Since the volume changes produced experimentally appear to reflect properties of the DNP, it has been suggested [9, 321 that chromosomal contraction at prophase is produced by a specific DNP-condensing agent possessing positively charged groups. Basic polypeptides, protamine, histone, or a polyamine such as agmatine, spermidine, or spermine were proposed as