Which of the following is not used in polymerase chain reaction?

PCR is an in vitro DNA amplification procedure in which millions of copies of a paicular sequence of DNA can be produced within a few hours.The reaction cycle has the following steps:Step 1: Separation (Denaturation): DNA strands are separated (melted) by heating at 95degC for 15 seconds to 2 minutes.Step 2: Priming (Annealing): The primers are annealed by cooling to 50degC for 0.5 to 2 minutes. The primers hybridize with their complementary single-stranded DNA produced in the first step.Step 3: Polymerization: New DNA strands are synthesized by Taq polymerase. This enzyme is derived from bacteria Thermus aquaticus that are found in hot springs.The steps of 1,2 and 3 are repeated. In each cycle, the DNA strands are doubled. Thus, 20 cycles provide for 1 million times amplifications. These cycles are generally repeated by automated instrument, called Tempcycler.5. After the amplification procedure, DNA hybridization technique or Southern blot analysis with a suitable probe shows the presence of the DNA in the sample tissue.Dideoxyribonucleotides are not used in the polymerase chain reaction.