A segment of DNA can be rapidly copied, or amplified, thousands of times with a laboratory technique called PCR or polymerase chain reaction. The DNA of interest is mixed with a heat-resistant form of DNA polymerase, the four types of nucleotides, and primers. The primers are synthetic strands of nucleotides which are designed to base pair with the ends of the DNA. The mixture is heated to separate the DNA strands and then cooled so that the primer binds to one end of the DNA. Now DNA polymerase, beginning at the primer, adds complementary nucleotides along the single-stranded DNA. The DNA sequence has now been doubled. The process is repeated as the mixture is alternately heated and cooled and the quantity of DNA doubles again and again. Each cycle of heating, cooling, and DNA doubling takes about 5 minutes. After 20 cycles there will be 1,048,576 copies of the DNA molecule. This technique has revolutionized molecular biology in such areas as diagnosis of disease, study of genetic disorders and forensic medicine.

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