FIG. 6. PCR. (1) Double-stranded DNA is heat-denatured in the first step of PCR. (2) Oligonucleotide primers, complementary to sequences flanking the target DNA sequence on opposite strands, anneal to the denatured DNA. (3) DNA synthesis proceeds in the 5' to 3' direction from the 3' terminus of each primer by the action of thermostable Taq DNA polymerase. (4) Newly synthesized DNA strands undergo denaturation, annealing of excess primers, and extension by DNA polymerase. Note the formation of short products that are of known base pair length and that multiply exponentially with each PCR cycle.