CHLOROPLAST DNA FROM BROAD BEAN

Chloroplasts purified by centrifugation through the glycerol density gradients of James and Das19 followed by the flotation technique of Jagendorf20 were free from nuclear contamination. DNA was isolated from pure chloroplasts and nuclei of broad bean. Some physicochemical properties, buoyant density, thermal denaturation and renaturation, Tm, base composition, sedimentation coefficient and molecular weight, of these DNA's were compared. Both chloroplast and nuclear DNA's have similar buoyant densities of 1.696 g/cm3 but only chloroplast DNA renatured readily. This difference in renaturation behaviour was used as a criterion for distinguishing one from the other and, consequently, the degree of purity of chloroplast preparations. Quantitative estimations made from pure chloroplasts gave a value of 5.7·10-15 g DNA per chloroplast or 0.33% relative to chlorophyll concentration. Comparing this value to the DNA content of the nucleus (5.8·10-11 g), it would mean that the DNA content of one nucleus is approximately equivalent to 1·104 chloroplasts. © 1969.