One of my favorite things to do with a student the first time they work with DNA plasmid preps is to have them run an agarose gel containing 2 samples: uncut plasmid DNA, and plasmid DNA that has been linearized with an restriction enzyme. I love to have them try and figure out the banding pattern of uncut plasmid DNA (why do you see 2-3 bands?) versus the single band of linearized DNA (and why do all bands convert to 1 band when linearized?).
I like this exercise because understanding the forms they are seeing on the gel requires an understanding of the nature of DNA. In this article I will focus on the four most common species of plasmid DNA observed on a gel and how to recognize them. And in my next article I’ll cover how to increase recovery of the desired supercoiled species.
One Molecule, Many Forms
When plasmid DNA is isolated and run on an agarose gel, you may observe 2, 3 or even 4 or more bands. Hopefully the majority of your isolated DNA will be supercoiled DNA, but other forms can also crop up. How these forms will show up on an agarose gel (in terms of relative migration speeds) is shown in the diagram below.
And here’s where each form comes from:
Supercoiled DNA is the native confirmation found in vivo and occurs when extra twists are introduced into the double helix strand. People often compare the forms of DNA to rubber bands or telephone cords (I know some you must still remember phones with cords!). If you over twist a rubber band or telephone cord, coils stack up upon one another introducing tension. In the case of plasmid preps, this superhelical tension cannot be relieved because the ends of the plasmid are joined together. Supercoiled DNA migrates faster than predicted in an agarose gel due to its conformation. Supercoiled DNA is the desired species when isolating plasmid DNA.
Nicked, Relaxed Circular Plasmid
DNA found in the supercoiled form is not easily accessed by replication machinery. During replication, cellular topoisomerases nick one strand of the DNA helix and relax the superhelical tension, thus allowing polymerases to gain access to the DNA. Using the rubber band analogy, nicked circle DNA is the rubber band without any twists introduced. This large floppy circle is the slowest migrating form in an agarose gel.
Linearized DNA occurs when the DNA helix is cut in both strands at the same place. Linear DNA generally migrates between the nicked circle and the supercoiled forms. However, it may also migrate the same distance as nicked circle — it migrates as predicted by the length of the DNA (as compared to the MW markers). You can identify the linear DNA form on an agarose gel by comparing uncut plasmid DNA with a sample of the plasmid that has been linearized using a restriction enzyme. If you get linear DNA when you are hoping for supercoiled (e.g. after a plasmid prep) it is due to nuclease contamination or harsh treatment during purification.
Circular, Single Stranded Plasmid
During alkaline lysis plasmid preps, plasmids are denatured because the hydrogen bonds are disrupted by the alkaline conditions. But the covalently-closed circular strands remain intact and topologically constrained and when the pH is returned to neutral the hydrogen bonds reform and the supercoiled DNA is re-formed. However, if the alkaline lysis step is overly harsh (e.g. it is incubated for too long) the DNA can become permanently denatured and give you useless single stranded closed circles that migrate ahead of all of the other forms of the plasmid in a gel.
Although DNA plasmid preps can return multiple forms of DNA, there is only one kind you want for successful cloning and transfection: supercoiled. Make sure you know how to increase your recovery of good quality supercoiled DNA.