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 ± 0.001 g per cm3 showed a Tm of 84° and a homogeneous melting pattern. Denatured and sheared ct-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 106 by renaturation rate, assuming a value of 106 x 106 for T4 DNA. The lysed chloroplasts are shown to contain 37% of the total DNA in circular molecules of 37 to 42 µ. The remaining DNA is found to be in linear form ranging from 1 to 30 µ. 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. φX174 RF II DNA and circularized phage λ DNA have been used as internal standards, and the molecular weight of ct-DNA is calculated to be 91 x 106.