Isolation and molecular weight of circular chloroplast DNA from Euglena gracilis

Abstract

1. Chloroplast DNA from Euglena gracilis has been examined by electron microscopy and by sedimentation analysis in order to establish its molecular size and in order to determine the feasibility of physical isolation of whole molecules. 2. Using bacteriophage λ DNA as an internal standard, a molecular weight of 9.2 · 10 7 is obtained for chloroplast DNA by comparison of contour lengths of circular λ and circular chloroplast DNA molecules found on identical grids in the electron microscope. Chloroplast DNA molecules were also found in a tightly twisted configuration; this is the first demonstration of supercoiled DNA structures in chloroplast preparations. 3. Sucrose gradient sedimentation analysis of chloroplast DNA with molecular wholes and halves of bacteriophage T7 DNA gives a molecular weight for chloroplast DNA identical to that obtained by electron microscopy. Furthermore, molecular whole and half molecules of chloroplast DNA can be effectively isolated by sucrose gradient sedimentation.