It depends upon if the circular plasmid is supercoiled or not. Generally undigested plasmids will mostly be in the supercoiled state and these will run with much faster mobility (ie appear smaller) than the linearized plasmid. However if the circular plasmid is not supercoiled but is nicked, then the mobilities will be similar. In most cases when you are comparing digested vs undigested then the plasmid will be supercoiled so with faster mobility.

Michael J. Benedik Thank you very much for the clearance.

I agree with MIchael J. Benedik, uncut plasmid DNA usually runs faster than cut plasmid DNA. Also, you usually expect the cut linear DNA to run as a single band, but uncut (circular) DNA can be a mixture of supercoiled and nicked so you will often see 2 bands for uncut DNA. If you plasmid is quite large the uncut bands will run close together and may or may not resolve (depending on many factors, plasmid size, gel %, run time, gel length, amount of DNA etc.)

Hope this helps!

Amy Klocko It sure helps. Thank you for the explanation.