Name and Chemical Components of PCR

The name 'Polymerase Chain Reaction' represents the nature of the process. 'Polymerase' because DNA Polymerase III is required for constructing new DNA strands, just like in a living cell. 'Chain Reaction' describes repeating cycles of replication which target a specific segment of a chromosome and use a “copy the copies” progression each cycle that doubles the amount of DNA copies of a specific segment of DNA present each cycle. In just 20 cycles of the chain reaction, over one million (220) copies of that specific segment of DNA can be produced. This is enough DNA to actually see with your naked eye. The goal of PCR is to make millions of copies of a specific segment of DNA that all originate from a single DNA sample.

The five chemical components that must be added to a test tube for the PCR reaction to work, include a DNA template, DNA polymerase III enzyme, single stranded DNA primers, nucleotides and reaction buffer.

  1. The DNA template is a sample of DNA that contains the target sequence of DNA for copying.
  2. DNA pol III. There are two requirements for a suitable DNA polymerase enzyme for PCR. First, the enzyme must have a good activity rate around 75°C. Second, the enzyme should be able to withstand temperatures of 95-100°C without denaturing and losing activity.
  3. Two primers. Primers are short oligonucleotides of DNA, usually around 20 base pairs in length. Because the purpose of PCR is to amplify a specific section of DNA in the genome, such as a known gene, then primers of specific sequences must be used. The geneticist planning the PCR reaction will design a forward primer to bind to one strand and a reverse primer that complements and binds to the other strand. The primer design process to select forward and reverse primers requires appropriate genetics thinking and is described later in this reading.
  4. The four different deoxyribonucleotide triphosphates (dNTPs). Adenine (A), guanine (G), cytosine (C), and thymine (T) are needed to provide the building blocks for DNA replication. DNA polymerase will add each complementary base to the new growing DNA strand according to the original strand’s sequence following normal A-T and C-G pairings.
  5. Finally, a reaction buffer. This creates a stable pH and provides the Mg2+ cofactor needed for DNA pol III activity.
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