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What is PCR?
PCR is a widely used technology in molecular biology and biomedical science. Through a series of heating cycles that pull apart, duplicate, and fuse back together the two strands of a DNA double helix, PCR creates new copies of DNA that are identical to the original.
How does PCR work?
Traditional PCR works in 3 steps, with each step sets at a different temperature. First, the sample is heated to 95°C, at which point the 2 strands of DNA separate from each other, or "denature." Then, the sample is lowered to 65°C so the separated DNA strands fuse or "anneal" with primers, which are short DNA strands designed to bond to specific locations of the target DNA sequence. Lastly, the sample is raised to 72°C to activate an enzyme called polymerase that attaches itself to the end of the primer. The polymerase "extends" the primer's length based on the original DNA, thus recreating the desired sequence.
During this "extension" step, the polymerase reacts with the original copy to produce a new chain of DNA, thus the name Polymerase Chain Reaction. Since this process happens for both of the original DNA strands, the amount of DNA is doubled after one PCR cycle. Over many cycles, PCR exponentially expands the amount of DNA you were started with.
Why is PCR useful?
PCR has an incredible range of uses. One of the most fundamental applications is in medical diagnostics, and it is also one of the easiest to understand. Imagine that you are looking for a specific DNA sequence that represents a disease. In a small drop of blood, there may only be very few copies of this DNA in a sea of other random DNAs, making it incredibly difficult to find the disease DNA - a classic needle-in-a-haystack problem. With PCR, many copies of the needle are created, until eventually finding the needle, and the disease, is a piece of cake.