Fig. 2.

Composition of DNA samples determined by electrophoresis. DNA samples were analyzed by polyacrylamide gel electrophoresis (5% polyacrylamide) in 150 mM NaCl and 10 mM CaCl₂ (to replicate the conditions used in analytical ultracentrifugation). Mr: 100 bp DNA ladder (linear DNA, lengths in bp as indicated). 333 and 339 bp minicircle topoisomer markers ($\mathrm{\Delta }Lk$ as indicated) were also loaded. Negatively supercoiled (Sc), linear (Ln), and nicked (Nc) 333 bp and 339 bp DNA samples were analyzed. The identity of the topoisomers in the supercoiled samples was determined by comparison to the topoisomer markers