FIGURE 1.

Representation of the PCR process. A dsDNA fragment is first heated (a) to break the molecule into its two complementary fragments (b). The solution is then cooled down to allow the primers to bind to their complementary sequences along the DNA fragments (c). Finally the solution is reheated to allow the polymerase to add nucleotides at the end of the primers and eventually make a complementary copy of the template (d). Because the polymerase can only act on one end of the DNA (the 3′ end), the solution quickly consists almost exclusively of DNA fragments corresponding to the target sequence located between the two selected primers (e).