What Is PCR (Polymerase Chain Reaction)

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∟What Is PCR (Polymerase Chain Reaction)

Provides a quick introduction of PCR (Polymerase Chain Reaction, which is a chemical synthesis process developed by Kary Mullis to rapidly amplify sequences of a DNA.

Sanger Sequencing method is based on PCR (Polymerase Chain Reaction) process, so let's look it first.

What Is PCR (Polymerase Chain Reaction)? - PCR (Polymerase Chain Reaction) is a chemical synthesis process developed by Kary Mullis to rapidly amplify sequences of a DNA.

PCR requires 3 inputs:

The original double stranded DNA.
Many of oligonucleotide primers, used to bind new DNA starting strands to both ends of existing DNA strands.
Lots of dNTPs (dATP, dCTP, dGTP, and dTTP), used to grow new DNA strands.

PCR is a multi-cycle process, starting from a single double stranded DNA. It doubles the number of DNAs in each cycle. In 35 cycles, 34 billion double stranded DNAs will be generated as shown in the diagram (source: utoledo.edu) below:

PCR Process Doubles DNA Strands Each Cycle

If you look at a single double stranded DNA in a PCR cycle, you will see that it gets replicated into two double stranded DNAs in 3 main steps:

1. Denaturation - The original double stranded DNA is denatured (or decoupled) into two complementary DNA strands.

2. Annealing - 5' ends of oligonucleotide primers are annealed to 3' ends of both DNA strands.

3. Extension - 3' ends of oligonucleotide primers are extended with a dNTP to grow into two double stranded DNAs.

The diagram (source: utoledo.edu) below illustrates the 3 steps of the PCR cycle:

PCR Reactions - Denaturation, Annealing, Extension

The diagram (source: macmillanhighered.com) below illustrates how a dNTP is synthesized to the 3' end of the new DNA strand in the extension step. In the diagram, the dNTP is actually a dCTP, which complements nucleoside base G on the template (target) strand at the next position of the DNA sequence.