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Imagine that you are a forensics investigator and you have been called to a crime scene to look for any potential evidence a criminal may have left behind. You carefully analyze the scene of the crime and find a single drop of blood on the windowsill where the culprit cut himself while breaking the window.

What can you do with this single drop of blood? If you have enough DNA, you can create a DNA fingerprint that may allow you to identify the person responsible for the crime. But a single drop of blood does not contain enough DNA to create a fingerprint. What can you do?

The answer is to use PCR (Polymerase Chain Reaction) to amplify (or make many copies of) the DNA.

PCR is a laboratory technique that uses the basic steps and materials of DNA replication to create a large number of copies of a small piece of DNA in a short period of time (PCR cannot be used to copy entire chromosomes). Invented in 1983 by Kerry Mullis (who won the Nobel Prize for his invention in 1993), PCR has revolutionized the study of biology and its applications.

PCR involves using a machine called a thermocycler to repeatedly heat and copy a segment of DNA. Added to the thermocycler are the template DNA, specific DNA primers, DNA polymerase, and DNA nucleotides (along with some other chemicals that help the reactions run).

The basic steps of PCR are:

1. Denaturation : the DNA is heated to separate the two complimentary strands (note that in a cell helicase would play this role).
2. Annealing : the temperature is lowered and the DNA primers attach (or anneal) to the regions around the segment of DNA to be copied.
3. Elongation : DNA polymerase copies the template segment of DNA.
4. Repeat the process : The process is repeated until the desired amount of DNA is obtained. In a few hours, millions of copies of a piece of DNA can be created.

The earliest generations of this technology were relatively slow and labor intensive because the denaturation stage would destroy the DNA polymerase (DNA polymerase, like all proteins, is heat sensitive). A technician would need to add new DNA polymerase after each heating cycle. However, in 1976 biologists had discovered a heat resistant DNA polymerase from a bacterium that lives in hot springs (Thermus aquaticus ). This DNA polymerase is now referred to as Taq polymerase and its use in PCR has made the process faster and less labor intensive because the Taq polymerase survives the heating cycles.

Yet another example of biodiversity playing an important role in improving human lives!

This short video (from 2 Minute Classroom) reviews the basics of PCR:

To watch this video on YouTube (and see closed captioning) - press the arrow icon in the bottom right corner of the video player.