When analysing a gel after running an undigested sample, why do you get multiple bands, if the sample is undigested? I understand that when you run a digested sample, you get many clear bands, but with undigested, you get a few faint bands, what do these represent?



The undigested sample contains fragments of DNA that are different sizes.  Therefore, when the gel is run, different sized bits of DNA run to different positions and you may see a smear of DNA. If the sample contains many copies of a particular fragment, or many fragments that happen to be around a certain size, this may appear as a faint band within the smear.

In essence, the undigested sample does not contain mutliple copies of DNA of one fixed length. The undigested sample has a mixture of DNA fragment sizes.

Best wishes,


True but the main reason is that supercoiled DNA (which is what one mostly has in a plasmid prep that has not been digested) will as part of the generation of the DNA have varying degrees of nicking. That variability in the degree of super-coiling will run at different rates on an agarose gel - hence the multiple bands/smear.

See also a previous post - http://www.askabiologist.org.uk/answers … hp?id=6797