Studies on nuclei: I. Physical properties of deoxyribonucleoprotein from rat thymus in 1 M sodium chloride

1. 1. Treatment of dilute rat thymus breis with concentrated NaCl solutions produced viscous solutions that were permanently disrupted by shearing. Attempts to measure the viscosity of such extracts in ordinary capillary viscometers were unsuccessful; however, reproducible measurements of apparent relative viscosity (± 2–3 per cent) were obtained by timing the first passage of the extracts through a modified Cannon-Fenske viscometer. 2. 2. The viscosity of extracts prepared in 1 M NaCl reached a maximum and constant value only after 10–12 hours of extraction at room temperature (25–27 °C). 3. 3. The maximum apparent relative viscosity of the extracts varied logarithmically with homogenate concentration. 4. 4. The viscosity of the extracts was markedly affected by pH. From pH 6.4 to pH 8.5, the viscosity was relatively constant; below pH 6.4, it fell rapidly, and, at lower pH's, was the same as for the solvent. Above pH 8.5, the specific viscosity increased rapidly to a maximum of about pH 10 and then dropped to zero at pH 11.5. 5. 5. The viscosity of the extracts was low in 0.05 to 0.4 M NaCl concentrations but increased rapidly in concentrations of 0.6 to 0.9 M. The viscosity of the extracts continued to rise slowly as the NaCl concentration was raised to 2 M but then decreased when the concentration was raised above 2 M. 6. 6. Viscosity decreased when the extracts or the breis from which the extracts were prepared were frozen. 7. 7. The results indicate that at least two types of protein-DNA bonds exist. The first, which cross-links DNA-protein strands in the nuclear gelwork, is easily broken by 1 M NaCl. The second, which binds the DNA molecules end to end to form long strands, is not broken in the initial extract and accounts for the high viscosity. The strands are believed to be the fundamental units of chromosome structure.