FIGURE 2 Gel electrophoresis is a technique (described in Chapters 4 and 6) that can be used to separate macromolecules by size. Molecules migrate through a gel (here, from top to bottom) in response to an electric current, with small molecules migrating faster than larger ones. In the experiment shown here, the polymerization of 1-methyladenine nucleotides is shown by the increasing size of DNA fragments with time. The numbers on the right indicate the length of the polymer in number of nucleotides; the labels below the lane numbers indicate the duration of polymerization of each sample in hours (h) and days (d).