ARCHAEOLOGICAL CHEMISTRY—EXPLORING HISTORY WITH CHEMISTRY - Chemistry: Notable Research and Discoveries - page 178

PolymeraseChainreaction
Kary Mullis, the inventor of polymerase chain reaction (PCR),
earned a Ph.D. in biochemistry from the University of Cali-
fornia, Berkeley, in 1972. The late 1960s and early 1970s
were turbulent times in Berkeley, with Vietnam war protests,
civil rights activism, and certain people—“hippies”—who en-
gaged in controversial lifestyles and habits. Yet out of this
cauldron came Mullis, a young biochemist born in North Car-
olina. Mullis took a job as a DNA chemist in 1979 at Cetus
Corporation in Emeryville, California. A few years later, in
1983, Mullis developed a series of chemical reactions that
is now paramount in forensics, genomics, and archaeology.
PCR uses natural enzymes to copy DNA molecules. Cells
replicate their DNA when they divide, so that each daugh-
ter cell will have its own copies. The enzyme, called DNA
polymerase, works by attaching to a single strand of DNA
and then catalyzing the reactions that synthesize the com-
plementary strand, which joins the other stand to make a
double-stranded helix.
To copy a DNA segment, a solution containing the DNA
to be copied, along with plenty of spare nucleotides, is
heated so that any double-stranded helices separate (the
high temperature breaks the hydrogen bonds holding the
two strands together). The DNA polymerase then goes to
work. But since the DNA polymerase needs an initial double-
stranded segment to get started, PCR technicians include
short sequences of DNA in the mix; these short sequenc-
es, known as primers, are complementary to the beginning
of the segment to be copied. The PCR machine cools the
solution slightly so that the primers bind correctly, but not
enough for long strands to bind together. After a short pe-
riod of time, the DNA polymerase fi nishes, and then the solu-
tion is heated again so that the strands separate. The PCR
machine repeats this cycle as often as necessary. From even
a single piece of DNA, this technique is capable of generat-
ing millions of copies in a few hours.