11.2.1.5—
Circularity

A recent discovery is that, when precautions are taken to minimize shearing, a proportion of chloroplast DNA can be isolated as circles. Circular DNA has been reported from chloroplasts of Euglena (Manning et al., 1971), Pisum sativum (Kolodner & Tewari, 1972), Spinacia oleracea (Manning et al., 1972), Antirrhinum majus, Oenothera hookeri, and Beta vulgaris (Hermann et al., 1975). Figure 11.2 shows a molecule of chloroplast DNA from Spinacia oleracea. The significance of circularity in DNA is not known, but it is a useful property since it implies the molecule has not been degraded on isolation. The most interesting aspect of this finding is that in all the species listed the contour length of the majority of the circles is in the range 37–45 µm; this length of double-stranded DNA has a calculated molecular weight of 0.85–1.0 × 10⁸ daltons, which is in the same range as the kinetic complexity of chloroplast DNA (Table 11.2). This correspondence of length and kinetic complexity suggests that the genetic information carried by the chloroplast DNA is accommodated by the length of the circular molecule.

Figure 11.2
Open circular DNA molecule, contour length
44.7 µm, from chloroplasts of  Spinacia oleracea.
(Reprinted from Hermann et al.,  (1975) by courtesy of Elsevier.)