why a plasmid dna that has been cut with restriction enzyme apper to be supercoil instead of linear?

Answer 1

The DNA remains linear. The plasmid that the DNA has been inserted into is supercoil

Answer 2

Plasmid DNA that is cut, or even nicked on one strand, will loose all supercoils. You have not cut your DNA. Try another enzyme.
Are you sure its a supercoil. Or is it partially digested. Try another enzyme.
Always run next to uncut. That will tell you what the supercoils, if any, look like. Most health plasmid preps have supercoils...unless you have sheared the DNA from too much pipeting or harshness....

Answer 3

Lots of answers to this question. Are you sure that it is cut? What enzyme/buffer concentration are you using? What does your reaction set-up consists of? Are you running it side by side with un-cut DNA? If so, and it's exactly alike, then you're not cutting it- try this if you haven't. It will tell you where you're uncut and cut DNA is at so you can purify the cut DNA. The site could be methlyated.