Structure of Chloroplast DNA

Abstract

We had previously reported the isolation of circular chloro-plast (et) DNA molecules from pea leaves. Circular pea ctDNA was found to have a molecular weight of 90×10⁶ with no evidence of inter- or intramolecular heterogeneity. Recently we have extensively studied the size and structure of ctDNAs from pea, bean, spinach, lettuce, corn, and oats.² As much as 89% of the ctDNAs from these higher plants has been obtained in circular form. The DNA preparations were also found to contain circular and catenated dimers of the circular monomer. The molecular sizes of these circular ctDNA molecules relative to internal standards has been found to range from 85×10⁶ to 97×10⁶. The molecular size of these ctDNAs was also determined by renaturation kinetics and found to range from 82x10⁶ to 93×10⁶. The excellent agreement between the molecular weights of these circular DNA molecules obtained by electron microscopy and the molecular weight of the unique sequences of these ctDNAs determined by renaturation kinetics suggests that the sequence of a circular ctDNA molecule represents the entire information content of the ctDNA.