Primers are short pieces of DNA that are made in a laboratory. Since they're custom built, primers can have any sequence of nucleotides you'd like. In a PCR experiment, two primers are designed to match to the segment of DNA you want to copy. Through complementary base pairing, one primer attaches to the top strand at one end of your segment of interest, and the other primer attaches to the bottom strand at the other end. In most cases, 2 primers that are 20 or so nucleotides long will target just one place in the entire genome. Primers are also necessary because DNA polymerase can't attach at just any old place and start copying away. DNA Polymerase is a naturally occurring complex of proteins whose function is to copy a cell's DNA before it divides in two. The DNA polymerase in our bodies breaks down at temperatures well below 95 °C (203 °F), the temperature necessary to separate two complementary strands of DNA in a test tube.
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