Once you have found a colony which contains a recombinant plasmid with both genes for antibiotic resistance, you will grow up a tube of cells from that colony overnight. This will give you many millions of E. coli cells, so you can isolate the plasmid DNA and analyze it on a gel.

Depending on how many different size pieces of DNA are in your recombinant plasmid, you will see different results on your gel. Review the Bacteria Selection page to remind yourself of the difference between a "simple recombinant" plasmid and a "super recombinant" plasmid.

The gel below is an example of results you might see from your isolated plasmids. The labels across the top of the gel refer to what was loaded into each lane. The numbers along the right side of the gel are the size fragments (in base pairs) represented by the bands in the Marker Lane. (Note that the top two Marker Lane bands have not separated completely.)

Sample 1 and Sample 2 represent plasmids isolated from two different colonies. The "cut" lanes were cut with both BamHI and HindIII. The "uncut" lanes contain intact plasmids. (There are multiple bands in the uncut lanes because the circular DNA is present in multiple forms which travel at slightly different rates.)

Determine what type of recombinant plasmid was present in each of the two colonies. Based on the plasmid restriction enzyme maps, what size should each of the fragments in the "cut" lane bands be? Check these numbers against the Marker Lane. To see if your answers are correct, click on the gel. (Don't peek until you've come up with your own answers!)