Molecular size and conformation of chloroplast deoxyribonucleic acid from pea leaves.

The molecular size and conformation of chloroplast (ct-) DNA has been studied by renaturation kinetics and electron microscopy. Ct-DNA of buoyant density 1.698 f 0.001 g per cm3 showed a T,, of 84” and a homogeneous melting pattern. Denatured and sheared c&DNA renatured as a single kinetic class with no suggestion of repeating sequences. Ct-DNA is found to have a molecular weight of 95 x lo6 by renaturation rate, assuming a value of 106 X lo6 for T4 DNA. The lysed chloroplasts are shown to contain 37% of the total DNA in circular molecules of 37 to 42 p. The remaining DNA is found to be in linear form ranging from 1 to 30 p. Ct-DNA has been isolated by deproteinization and shown to contain 25 % of the DNA in circular form. Supercoiled molecules constitute 30% of the circular molecules. Three per cent of the circular DNA is found in circles of dimer lengths. 4X174 RF II DNA and circularized phage h DNA have been used as internal standards, and the molecular weight of ct-DNA is calculated to be 91 x 10”.