midi-prep - (Oct/24/2013 )

In my case, this kind of problem was due to the loss of the "toxic" plasmid during culture growth (in which I used ampicilin) and that's why I didn't get any plasmid DNA after midiprep. This is the reason why I asked you which antibiotic you were using.

I don't know if this is your case, but perhaps something like this could be happening to you: bacteria grow very well in culture... but getting rid of the plasmid. Then, when you do your midiprep, you get a very low plasmid amount... in any.

Other possibility, of course, is that you did something wrong during the midiprep protocol, but I assume that you did everything right...

Anyway, good luck! :)

how can I know either my plasmid is low copy number or high copy number???

I think pET16 is a high copy number vector. You can find this information in the brochure (if it's a commercial plasmid) and/or in the internet (just google it).