I conduct a thermal stability test for plasmid DNA. In which I put plasmid DNA under 65-degree celsius for about one week. After that, I run an agarose gel check.

Here below is what I got: 3 left (3 heated samples), 3 right (3 controlled samples). The control showed 2 bands. They might be Supercoiled form and Circular form I suppose. However, the 65-degree samples showed 2 closeby bands (the upper might be supercoiled). What is the lower band conformation? Could you drop some hint. Thank you ~