The stages in the Polymerase Chain Reaction (PCR) are depicted as follows:

1. DNA fragment to be copied is heated to separate strands.
2. The mixture is cooled, and primers bind to strands.
3. DNA polymerase adds nucleotides to strands, producing two complementary strands. At the end of the cycle two copies are produced.
4. The procedure is repeated starting at step 1.

The cycle completes here and two copies of DNA are obtained. Four copies are obtained by repeating the same procedure starting from heating of DNA.