Stability and fine structure of eukaryotic DNA rings in formamide

Folded rings formed from Drosophila and Necturus DNA fragments were examined by electron microscopy in increasing concentrations of formamide, in an effort to identify regions of non-homology within the closure region. Unusual closure regions of this type were not found, in spite of an extensive search. If such regions exist, they must be too short to be detectable (<50 nucleotides), or longer than 1000 nucleotides. In this latter case, they could not be contained within the overlap region of the ring. A study of the thermal (formamide) stability of these rings in relation to the observed closure lengths suggests that extensive (>2 to 3%) mismatching is not possible. At higher formamide concentrations, some rings will partly denature, yet remain circular because the closure region remains intact.