Pigs to the Rescue ■ 163DNA quickly in a test tube. In the 1980s, scien-
tists developed a way to mimic natural DNA re -
plication in a laboratory—fast. The polymerase
chain reaction (PCR) is a technique that makes
it possible to produce millions of copies of a
targeted DNA sequence—to “amplify” the DNA—
in just a few hours, even with an extremely small
initial amount of DNA (Figure 9.9).
PCR relies on heat, not special proteins,
to cause the DNA to unwind and its strands
to separate. PCR also uses targeted primers to
start the replication process at points deter-
mined by the scientist, rather than at DNA’s
natural origins of replication. The researcherto the template strand, to “proofread” the results,
and to join partly replicated fragments of DNA
to one another, ultimately forming a completely
replicated chromosome.
Despite the complexity of this task, cells
can copy DNA molecules containing billions
of nucleotides in a matter of hours—about
8 hours in humans (over 100,000 nucleotides
per second). This speed is achieved in part by
starting the replication of the DNA molecule
at thousands of different origins of replication
at once.
Before CRISPR was developed, the greatest
genetic technology was a tool for replicating
Primer New DNANew DNANew DNANew DNAOriginal DNA
sequenceHeat separates
the double
strands of the
original DNA
into two single
strands.As the
mixture cools,
the primers
pair with the
original DNA.DNA polymerase fills in the
missing nucleotides, producing
new copies of the original DNA.The same three-step cycle can be
repeated many times, yielding
billions of copies of the original DNA.Cycle 1 Cycle 2 Cycle 31 2
3
P P
P
P P
P
P
P
P
P
P
P
P
P
P
P
P
P
P P
P
P
P P
P
P
P
P
Figure 9.9
PCR can amplify small amounts of DNA more than a millionfold
Short primers consisting of synthetic DNA segments are mixed in a test tube with a sample of the target DNA, the enzyme
DNA polymerase, and all four nucleotides (A, C, G, and T). The primers form base pairs with the two ends of a gene of interest. A
machine then processes the mixture and doubles the number of double-stranded versions of the template sequence. The doubling
process can be repeated many times (only three cycles are shown here).
Q1: PCR replicates DNA many times to increase the amount available for analysis. Why is this process called
“amplification”?Q2: During the PCR cycle, what causes the DNA strands to separate?Q3: Identify a difference between how PCR and DNA replication are accomplished.