Full form of PCR is Polymerase Chain Reaction.

The following procedure is applied for obtain multiple copies of DNA. 

At the start of PCR, the DNA from which a segment is to be amplified, an excess of the two primer molecules, the four deoxyriboside triphosphates and the DNA polymerase are mixed together in the reaction mixture. The following operations are now performed sequentially. 

Step 1: The reaction mixture is heated to a temperature (usually 90-98°C) that assures DNA denaturation.

Step 2: The mixture is now cooled to a temperature (generally 40-60°C) that permits annealing of the primer to the complementary sequences in the DNA; these sequences are located at the 3 ends of the two strands of the desired segment. This step is called annealing.

Step 3: The temperature is now so adjusted that the DNA polymerase synthesizes the complementary strands by utilizing 3'- OH of the primers; this reaction is the same as that occurs in vivo during replication of the leading strand of a DNA duplex. The primers are extended towards each other so that the DNA segment lying between the two primers is copied.

The completion of step 3 completes the first cycle of amplification; each cycle may take few (ordinarily 1-3) minutes.

Step 4: The next cycle amplification is separated the newly synthesized DNA strands from the old DNA strands.

Step 5: Annealing allows the primers to base pair with both the new and did strands, the total number of strands being twice their original number.

Step 6: Synthesis of new strands takes place, which doubles the number of copies of the desired DNA segment present at the end of step 1. This completes the second cycle.

Thus at each cycle, both new and old strands anneal to the primers and serve as templates for DNA synthesis. As a result, at the end of each cycle, the number of copies of the desired segment becomes twice the number present at the end of the previous cycle. Thus at the end of n cycles 2n copies of the segment are expected; the real values are quite close to this expectation. The cycle may be repeated.