Which of the following does not occur during the PCR reactions?




A.Changes in the reaction temperature by a thermocycler



B.Denaturation at high temperatures



C.Synthesis of oligonucleotide primers



D.Extension of primers by DNA polymerase



E.Annealing of oligonucleotide primers

C. synthesis of oligonucleotide primers

The correct answer is C. Synthesis of oligonucleotide primers does not occur during PCR reactions. During PCR, the other steps listed do occur:

A. Changes in the reaction temperature by a thermocycler: The reaction temperature is altered at different stages of the PCR process to facilitate denaturation, annealing, and extension.

B. Denaturation at high temperatures: The first step of PCR involves heating the DNA sample to a high temperature to denature the double-stranded DNA into single strands.

D. Extension of primers by DNA polymerase: After denaturation, a DNA polymerase enzyme adds nucleotides to the primers, extending them and synthesizing new DNA strands.

E. Annealing of oligonucleotide primers: After denaturation, the reaction temperature is lowered to facilitate the binding (annealing) of the primers to the complementary sequences on the single-stranded DNA template.

Therefore, synthesis of the oligonucleotide primers is not a step that occurs during the PCR reactions.

To determine which of the following does not occur during PCR reactions, we need to understand each step of the PCR process. PCR stands for Polymerase Chain Reaction, which is a widely used method for amplifying specific DNA sequences.

Here is a breakdown of each option and its role in the PCR process:

A. Changes in the reaction temperature by a thermocycler: This is true and occurs during PCR. The thermocycler is an instrument used to control and cycle the temperature of the reaction mixture, which is essential for the different steps of PCR.

B. Denaturation at high temperatures: This step occurs during PCR. Denaturation involves heating the DNA sample to a high temperature (typically around 94-98°C). This high temperature causes the double-stranded DNA to separate into two single strands.

C. Synthesis of oligonucleotide primers: Oligonucleotide primers are short DNA sequences used to initiate DNA synthesis during PCR. They are designed to be complementary to the specific DNA region to be amplified. The synthesis of oligonucleotide primers does not directly occur during the PCR reaction, but they need to be prepared and added to the reaction mixture before starting PCR. Therefore, this option is the correct answer as it does not occur during PCR reactions.

D. Extension of primers by DNA polymerase: This step occurs during PCR. After denaturation and the addition of the oligonucleotide primers, DNA polymerase, an enzyme, extends the primers by adding complementary nucleotides to each single-stranded DNA template.

E. Annealing of oligonucleotide primers: This step occurs during PCR. After denaturation, the reaction temperature is lowered for the primers to anneal or bind specifically to their complementary DNA sequences. Annealing allows the primers to position themselves for DNA synthesis.

In conclusion, the answer is C. Synthesis of oligonucleotide primers, as this step does not occur during PCR reactions but rather before the PCR process begins.