Question

You digest a plasmid with a restriction enzyme that recognizes a single site on the plasmid. When you perform gel electrophoresis on the digestion product, you quickly realize that there are two bands; one at the expected size and one near the well. Which of the following best explains the outcome?

DNA was trapped in the agarose gel
The presence of the chromosomal DNA
Some plasmids were not digested
The some plasmids ligated together

Answer 1

You found 2 bands because the plasmid is not digested properly(3rd option)

Explanation -

There is no chance of any presence of chromosomal DNA, because if there was chromosomal DNA then there will be more fragments in gel as chromosomal DNA is so large and have more restriction site then plasmid. Also the breaking of chromosomal DNA leads to the formation of such fragments which can form more then 2 bands.

If the DNA is trapped in the gel then there will be only one band in gel.

Plasmid could not be ligated because in restriction digestion of plasmid, there is only restriction enzyme and no ligation enzyme for ligating plasmid.