• What is PCR? Describe the technique in detail.
  • What enzyme is used in this process and how is the enzyme used in PCR different from others?
  • What is that enzyme's function in the cell?
  • How has PCR changed microbiology?

Expert Answers

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PCR stands for Polymerase Chain Reaction.

PCR is a revolutionary technique that has allowed microbiologists to study particular regions of DNA, simply by making an enormous number of copies of that region; 30 PCR cycles will produce over a billion copies. Having many copies allows us to work around the difficulties of DNA's small size, as well as other problems such as having a very small sample to start with, e.g. forensics.

The enzyme used in this process is polymerase. Recall that, during mitosis, the cell must make a copy of its DNA so that both will have a full copy; this requires splitting the DNA helix open, and making complimentary copies for both of the single strands, before they coalesce into two separate helices. Polymerase is responsible for making the complimentary copy for the open strands.

In PCR, the DNA helices are separated artificially by heating them. Human polymerase would be destroyed at the temperature necessary to separate the DNA, so instead we use Taq polymerase, which we take from heat-resistant bacteria that live in hot springs. The Taq can tolerate the heat necessary to separate the DNA helix, and then go to work making copies.

However, we don't want to copy the entire DNA sequence; we just want certain parts of it. To identify these locations, we create primers. The primers are chains of nucleotides that are matched up to parts of the DNA strand that immediately precede the section we want to copy. The Taq, seeking the end of the primer, attaches and continues copying from there.

Repeated heating, primer attachment and polymerase synthesis produces the "chain reaction". Each reaction doubles the amount of DNA produced.

In my links I have included an excellent virtual tutorial on PCR reactions from the University of Utah.

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