Plasmid Migration on Gel

[eternalmetal]

I had an impurity in my sample after a BglII digestion followed by CIP. To attempt to assess the problem with a quick gel, I ran in the first lane the original plasmid straight from the miniprep, a sample containing the plasmid cut by itself, and in the last lane the sample cut with BglII and CIPed, showing the impurity at around 2kb. the marker is a 1kb ladder with bands at 10, 8, 6, 5, 4, 3, 2, 1.5, and 1k and 500 bp.

 

Here is a picture of the gel im talking about:

 

Now I know that when you run an uncut plasmid you typically get 3 bands. Nicked/open, linear, and supercoiled. The sample however contains those 3 bands, and a 4th unknown band. Note that when I cut with BglII, I am only supposed to be cutting at one restriction site, so there should only be one band. But when I cut this plasmid I get an unknown 2 kb band. My guess is that I have an impurity somewhere from the miniprep, but what could it be? Im using an E.Z.N.A. plasmid miniprep kit to prepare the plasmid.

 

Any insight is appreciated.

[CharonY]

I do not think that you got impurities. First, minipreps generally result primarily in OC, CCC plasmids and then you got multimers (but still closed). So additional higher MW bands are perfectly normal. Linear DNA tends to pop up if some mechanical nicking occurred. From the gel it appears that the additional band is only seen in the phosphatase treated sample? Because the digest alone looks perfectly fine (is it the expected size)?