A single DNA nucleotide, the base unit of the human genome, is made up of only 30 atoms (plus or minus a few, depending on the base), making it much too small to be identified by even the most powerful electron microscope. So how, then, do researchers determine the sequence of A's, G's, C's, and T's that comprise the genome?

The answer is that they've had to develop a new process, what Bruce Birren of MIT's Whitehead Institute calls a "biochemical magnifying glass." But this magnifier doesn't enlarge the view of DNA bases. Rather, it makes billions of identical copies of a small sequence of DNA -- enough copies to allow a machine to detect marked bases within the DNA.

At first glance, the method described in this feature may seem a bit complex, even roundabout. But then sequencing the human genome is not as easy as A-G-C. The concepts involved, however, are easy to grasp. "Sequence for Yourself" should give you a good idea of the process.