PCR stands for Polymerase Chain Reaction, a method for amplifying DNA in vitro. Where, in cloning DNA into a plasmid, the DNA is amplified by the bacterial cell when it replicates the plasmid, PCR amplifies DNA in a test-tube. Cycling temperatures: Of the three temperatures used in a cycle of PCR, the denaturing and extension temperatures remain unchanged for the most part, since most DNA is denatured at 94-95 degrees C, and the temperature optimum for the Taq DNA polymerase lies between 72 and 74 degrees C.