Initially general aspects of DNA structure, particularly tertiary, and its investigation are discussed together with the difficulties encountered; this leads to the choice of ϕX174-RF DNA, investigated mainly by light scattering as the experimental system for investigating DNA structure. As ϕX174-RF DNA is circular and exists in two forms, RFI which is supercoiled and RFII which is an open circle, the implication of this and the significance of the experimental work undertaken are discussed in the light of previous workers' findings. The following experiments were performed: (1) light scattering on DNA at 2
