Abstract
Maize seedling DNA has been characterized with respect to the reformation of double-stranded structures after heat denaturation. Two types of reversibility [Geiduschek, E. P. (1962) J. Mol. Biol. 4, 467–487] are demonstrated. Type II reversibility, the intermolecular reassociation of complementary sequences in completely separated strands, is followed as a decrease in absorbance. Type I, the intramolecular reannealing of partially separated strands, is demonstrated by three independent techniques (absorbance change, binding to nitrocellulose filters and thermal chromatography on hydroxyapatite). Type I reversibility in maize DNA is accentuated both in its ubiquity (half the base pairs are reformed) and in the stability (at 99 °C in 0.18 M Na +) of the nucleation sites responsible for this behavior. Data are presented which indicate that such stability is due to the presence of regions of greater than 70 % guanine-cytosine base pairs in DNA of overall base composition 46 % G+C . Such regions occur an average of once per 4800 base pairs, have a maximum mean length of 250 base pairs and are distributed among adjacent segments of both high and low G+C content.
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