This animation shows the process by which
part of a DNA molecule is copied using the polymerase chain reaction, or PCR.
The polymerase chain reaction derives its name from the DNA polymerase, which
is used to copy the DNA sequence of interest. This process of copying, or
amplification, can be carried out in small test tubes without the use of
cloning. To begin, the DNA to be amplified is melted, or separated into two
strands, by heating. For PCR to occur, two short segments of DNA known as
primers are needed. These are shown here in magenta and are complementary to
the beginning and end regions of the DNA sequence to be amplified. Keep in
mind that only the DNA found between these two primers must be copied. The
temperature is reduced slightly to allow the primers to bind, or anneal, to
the DNA. Then, the enzyme DNA polymerase replicates the two strands starting
at the position where each primer is annealed. This creates two new DNA
strands, one of which begins at the position of interest and another that
ends at the desired region. After additional rounds of amplification,
however, most of the DNA molecules produced are of the sequence length
desired, with no overhanging regions. In the second round the strands are
melted from one another once more, primers bind to the new as well as old
strands, and replication occurs again. Notice that the number of new strands
produced has doubled compared to the first cycle, and that two of the four
new strands are now of the correct length. The process is repeated again in a
third cycle – this time eight new DNA strands are produced, six of which
possess the correct sequence length. In addition, note that four of the
strands with the sequence desired have formed two double stranded molecules.
The amplification process is repeated for additional rounds, usually for
about 30 cycles, until most of the DNA molecules formed are double-stranded replicas
of the sequence of interest, and are present in large quantities, as shown.
These DNA segments, which typically contain functional genes, can then be
used in experiments involving the study of gene regulation and expression.
Using this process, over a billion copies of a DNA fragment can be produced
from a single double-stranded molecule if amplification proceeds for 30
cycles (230 =1073741824).
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