Biology’s equivalent of an office copier is a PCR machine. PCR, short for polymerase chain reaction, is now a staple in crime-scene forensics, heredity tests, and organism hijacking. It’s a mind-boggling feat. Among billions of base pairs that make up DNA’s genetic code, PCR finds exact sequences and, in a couple of hours, makes billions of copies-enough to decode or splice together useful combinations of genes. Professional machines cost about $10,000 dollars each, but Russell Durrett has devised one from PVC pipes, a 150-watt lightbulb, a computer fan, a cheap microcontroller, and a few reagents he ordered online.
1) PILE UP
Stack five pieces of PVC piping to form a silo [right] that holds vials of DNA, heats them with a lightbulb, and cools them with a computer fan.
Top: Drill a dozen or so ¼-inch holes into the top of a slanted adapter (to hold the vials), fit a pipe inside the adapter’s base, and cut a square hole in the exposed pipe to match a computer fan’s exhaust port.
Middle: Cut the same square port in the top of a straight PVC coupling and hot-glue the fan in place. Drill a hole into the coupling’s side, just above the center, and bolt on a clamped lightbulb socket.
Bottom: Drill a hole in a second coupling for a bundle of wires leading to an Arduino Uno microcontroller.
2) WIRE UP
Connect the socket and computer fan to 5-volt relays, and link them to the Arduino. Lead one socket wire to a 110-volt AC power source, and wire a thermistor to the Arduino. Insert the thermistor into a vial containing mineral oil and water, and place it in a vial hole. This gives the Arduino temperature data, allowing it to turn the lightbulb-and heat-on and off.
3) PICK A GENE
Whether you’re testing breakfast cereal for genetically modified grains or screening yourself for HIV resistance, pick a gene to copy. You’ll need to order from the Internet reagents that can hijack DNA-copying molecules [see “How It Works,” on the next page.]
HOW IT WORKS
PCR machines cycle temperature to enable DNA-copying reagents, including primers, nucleotides, and the enzyme Taq polymerase.
DNA temporarily unzips into two strands.
Primers-small pieces of DNA that “prime” replication-attach to the ends of a gene.
Taq polymerase recognizes the primers, latches on, and copies DNA between them.
. . . And Repeat
In the first five minutes, PCR makes two copies of a gene. Subsequent 30-second cycles continue the doubling. By the eighth cycle, 256 genetic copies exist, and by the 30th-
a couple of hours later-there are more than a billion.
Time: 5 hours
This article originally appeared in the May 2013 issue of Popular Science. See the rest of the magazine here.
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