The polymerase chain reaction, or PCR, amplifies a specific DNA fragment from a complex mixture.First, the mixture is heated to separate the DNA strands. Two different specific oligonucleotide primers are added that are complementary to short sequence stretches on either side of the desired fragment. After lowering the temperature, the primers hybridize to the DNA where they bind specifically to the ends of the desired target sequence. A heat stable DNA polymerase and nucleotide triphosphates are added. The polymerase extends the primers and synthesizes new complementary DNA strands. At the end of this first cycle, two double-stranded DNA molecules are produced that contain the target sequence. This cycle of events is repeated. The mixture is again heated to melt the double-stranded DNA. The primers are hybridized and the DNA polymerase synthesizes new complementary strands. At the end of the second cycle, four doubled-stranded DNA molecules are produced that contain the target sequence. In the third cycle, the mixture is heated, the primers are hybridized and DNA polymerase synthesizes new complementary strands. At the end of the third cycle, eight double-stranded DNA molecules are produced that contain the target sequence. Two of these molecules are precisely the length of the target sequence. In future cycles this population increases exponentially. Cycle 4--heating, hybridization, DNA synthesis. At the end of the fifth cycle there are 22 double-stranded DNA fragments of the correct length and 10 longer ones. Cycle 6, 10, 15, 20 . . . After 30 cycles there are over 1 billion fragments of the correct length but only 60 longer ones. The product therefore consists of essentially pure target sequence.