TABLE OF CONTENTS In 1963 by Dulbecco and Vogt [1] and Weil and Vinograd [2] discovered that double-stranded DNA of the polyoma virus exists in a closed circular form. At present it is generally acknowledged that this form is typical of bacterial DNA and of cytoplasmic DNA in animals. Furthermore, giant DNA molecules in higher organisms form loop structures held together by protein fasteners in which each loop is largely analogous to closed circular DNA. The distinctive feature of closed circular molecules is that its topological state cannot be altered by any conformational rearrangement short of breaking DNA strands. This topological constraint is the basis for the characteristic properties of closed circular DNA which have fascinated biologists, physicists and mathematicians for the past 35 years. In this chapter we concentrate our attention on physical properties of circular DNA which understanding is necessary for the successful analysis of the biological effects connected with this form of DNA.
Two forms of circular DNA molecules are extracted from the cell; they were designated as form I and form II. The more compact form I was found to turn into form II after a single-stranded break was introduced into one chain of the double helix. Subsequent studies performed by Vinograd et al. [3] linked the compactness of form I, in which both DNA strands are intact, to supercoiling. Form I came to be called...
