Arrange the following in the proper sequence in which they occur during a single PCR cycle.

1. Addition of DNA nucleotides by Taq DNA polymerase.
2. Complementary base pairing between primers and target DNA.
3. Heat separation of strands of target DNA.
A. 1, 2, 3
B. 3, 1, 2
C. 3, 2, 1
D. 2, 3, 1
E. 2, 1, 3

During a single PCR cycle, the very first step is the heat separation of the double stranded DNA. Primer binds only single stranded DNA/RNA because it has complimentary bases which match single strand of DNA/RNA that is why it is necessary to unwind both the strands of DNA by providing temperature.

As soon as the DNA strands have separated by increasing the temperature to ~ 95 °C, in the next step the primer binds the portion of DNA which is complimentary to the bases of the primer. In order for primer to bind DNA, the temperature is lowered to ~ 72 °C and the process is called as annealing.

Once primer has bound to the DNA, the next step is polymerization i.e. nucleotides which are the monomer units of DNA are added one by one with the help of enzyme named as Taq polymerase. This is how the strand extends ahead of primer and exact copies of the given DNA are produced.

Note: PCR may be considered as in vitro DNA synthesis or replication.