The polymerase chain reaction (PCR) is a procedure that borrows a cellís mechanism for DNA replication, allowing researchers to make many copies of a gene of interest. Starting materials are
A simple test procedure rapidly builds up copies of the gene. Here's how it works.
In the first step of PCR, heat is used to separate the two strands of the target DNA. Next, the temperature is lowered and the two short DNA primers are added. The primers bind by complementary base pairing to the separated target strands. In the third step, DNA polymerase and bases are added. The enzyme adds bases to the primers and builds a sequence complementary to the target sequence. The newly synthesized strands then act as templates in the next round of replication, which is initiated immediately by raising the temperature. All of this is done in an automated device called a thermal cycler that controls the key temperature changes.
The pieces of DNA accumulate geometrically. The number of amplified pieces of DNA equals 2n , where n equals the number of temperature cycles. After just twenty cycles, one million copies of the original sequence float in the test tube.
PCR's greatest strength is that it works on crude samples of rare and minute sequences, such as a bit of brain tissue on the bumper of a car, which is one criminal case lead to identification of a missing person. PCR's greatest weakness, ironically, is its exquisite sensitivity. A blood sample submitted for diagnosis of an infection contaminated by leftover DNA from a previous run, or a stray eyelash dropped from the person running the reaction, can yield a false positive result. The technique is also limited in that a user must know the sequence to be amplified.