PCR is a technique for isolating and duplicating DNA fragments. It was invented in the 80s and Kary Mullis the creator won the 1993 Nobel Prize for it. It's quick and cheap and has many uses and is one of the linchpins of modern genetics. Here's a good flash movie explaining PCR. There aren't that many Nobel prize worthy things you can understand in a few minutes but this is one of them.
I would change a few things about the presentation. I'd move earlier the explanation that nucleotides pair up as A with T and G with C. Also I'd explain that the enzyme walks the DNA chain in just one direction. The primers are deliberately chosen to represent the two ends of the specific sequence you're interested in duplicating, and must be long enough to be unique across the whole strand of DNA (usually 20 base pairs is enough). While this video does a good job of showing how some duplicated DNA has extraneous trails, it took me a few times thinking it through of how you get duplicates of just the material you're interested in.
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