A photo finish

A gel run can last hours so a molecular geneticist only views the finish of the DNA fragment race. The gel is run for a set period of time and then the stained gel is photographed. The timing of this photo is important to properly compare the lanes of DNA segments. If the race is ended too early, the smaller segments may not have enough time to get ahead of the larger segments (Fig. 19). If the race is ended too late, the smaller segments may have already reached the end of the gel and run off into the gel box. However, if the gel run is ended at the correct time, DNA samples with shorter fragments can be pictured forming bands that have moved farther through the gel than samples of longer fragments (Fig. 20). When a DNA fragment banding pattern is different between samples, genetic differences are inferred to exist between the sources of each sample. The geneticist will describe this DNA fragment pattern as polymorphic (many forms). If the fragment banding is the same among samples they are monomorphic and no genetic differences are inferred from this analysis. So, whether a DNA race ends in a tie or whether fragments move at different speeds, gel electrophoresis reveals genetic information.

Fig. 19. This gel has not run long enough to resolve two fragments. (Image by D. Lee)

Fig. 20. Running the gel longer resolves the DNA sample into two bands. (Image by D. Lee)