|MadSci Network: Molecular Biology|
Molecular biologists (of which, I must confess, I am not one) have the coolest collection of clever tricks of any major profession today. If you keep asking this question, you'll hear of a lot of amazing techniques. Scientists studying DNA are usually looking not at single molecules but at large numbers of molecules. By staining DNA with fluorescent dyes, you can tell (roughly) how much DNA is at various spots on a thin sheet of jellylike agarose gel. If you push the DNA down the sheet of gel with an electric field, the shorter pieces move faster than the longer pieces. So now you can tell how much DNA you have in pieces of various lengths. "Restriction enzymes," discovered in bacteria, snip DNA molecules at places where the sequence of DNA bases matches some particular pattern. If you expose some DNA to a restriction enzyme and then "run it on a gel," the lengths of the DNA snippets tell you the spacings between the patterns that the restriction enzyme recognizes. If you start making a lot of copies of a piece of DNA, but "poison" the process with a poison that will sometimes stop the copying when a Guanine base is being added to the new DNA molecule, the lengths of the resulting strands tell you exactly how far from the beginning of the DNA molecule the Guanine bases appear. And so forth, with trick after trick. If you want even cleverer tricks, ask about the Polymerase Chain Reaction.
Try the links in the MadSci Library for more information on Molecular Biology.