Circular DNA

In 1963 by Dulbecco and Vogt (1963) and Weil and Vinograd (1963) discovered that doublestranded 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 40 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 the molecule.