Base composition of DNA from chloroplasts and nuclei of Phaseolus vulgaris.

Abstract

WHEN the first reports of the isolation and biochemical study of chloroplast DNA from higher plants appeared in 1963, there was a discrepancy between the two sets of findings. J. T. O. K.1,2, using chemical analysis, reported that broad bean chloroplast DNA had a guanine-cytosine content (37.4 per cent GC) close to, but slightly (and significantly) lower than, that of the nuclear DNA (39.4 per cent GC). Chun, Vaughan and Rich3 using spinach and beet, and later other workers using swiss chard4, tobacco5 and other plants6, reported, however, that DNA from chloroplast preparations analysed by CsCl equilibrium density gradient centrifugation contained—in addition to a component of similar density to nuclear DNA—one or two components of much higher density (and therefore of much higher GC content): these dense components they considered to be the true chloroplast DNA. Recent and more rigorous studies7–9 have now cast considerable doubt on the identification of these high density bands with chloroplast DNA.