Molecular Properties of Chloroplast and Nuclear DNA from Mimulus cardinalis

Abstract

Summary DNA was isolated from nuclei, chloroplasts and whole cells of the angiosperm Mimulus cardinalis and characterized as to mole % guanine plus cytosine (G + C) and for the presence of variation in intramolecular heterogeneity in base composition. Isolation was accomplished via hydroxylapatite chromatography and characterization by thermal denaturation spectrophotometric analysis. The mean T m values obtained from the thermal denaturation profiles for whole cell DNA, nuclear DNA (nDNA) and chloroplast DNA (cDNA) are, respectively, 67.32 °C, 67.20 °C and 68.90 °C. The respective G + C contents calculated from the T m values are 32.75%, 32.45% and 36.60%. When per cent hyperchromicity was plotted against temperature on normal probability graph paper, whole cell and organelle DNA exhibited the presence of three distinct hyperchromic fractions, i.e., an adenine plus thymine — A + T — rich fraction, a main band, and a G + C- rich fraction. These three fractions are interpreted to reflect the presence of a non-Gaussian distribution of bases and thus intramolecular heterogeneity within nDNA and cDNA. nDNA and cDNA are highly resoluable by this technique. The per cent of the total rise in hyperchromicity attributable to the A + T-rich, main band, and G + C-rich fractions in nDNA and cDNA are, respectively, 16.5%, 44%, 39.5% and 17%, 72.7%, 10.3%.