The twisted circular form of polyoma viral DNA.

Discovery of DNA supercoiling. DNA supercoiling refers to the amount of twist in a particular DNA strand, which determines the amount of strain on it. A given strand may be "positively supercoiled" or "negatively supercoiled" (more or less tightly wound). The amount of a strand’s supercoiling affects a number of biological processes, such as compacting DNA and regulating access to the genetic code (which strongly affects DNA metabolism and possibly gene expression). With R. Radloff, R. Watson & P. Laipis. Digital facsimile from PubMedCentral at this link. See also, Jacob Lebowitz, "Through the looking glass: The discovery of supercoiled DNA," Trends in Biochemical Sciences, 15 (1990) 202-207.