PCR (Polymerase Chain Reaction) (cont.)
How is PCR (polymerase chain reaction) done?
As illustrated in the
animated picture of PCR, three major steps
are involved in a PCR. These three steps are repeated for 30 or 40 cycles. The
cycles are done on an automated cycler, a device which rapidly heats and cools
the test tubes containing the reaction mixture. Each step -- denatauration
(alteration of structure), annealing (joining), and extension -- takes place at
a different temperature:
- Denaturation: At 94 C (201.2 F), the double-stranded DNA melts and opens into two
pieces of single-stranded DNA.
- Annealing: At medium temperatures, around 54 C (129.2 F), the primers pair up
(anneal) with the single-stranded "template" (The template is the sequence of
DNA to be copied.) On the small length of double-stranded DNA (the joined primer
and template), the polymerase attaches and starts copying the template.
- Extension: At 72 C (161.6 F), the polymerase works best, and DNA building blocks
complementary to the template are coupled to the primer, making a double
stranded DNA molecule.
With one cycle, a single segment of double-stranded DNA template is amplified
into two separate pieces of double-stranded DNA. These two pieces are then
available for amplification in the next cycle. As the cycles are repeated, more
and more copies are generated and the number of copies of the template is
increased exponentially.
What is the purpose of doing a PCR (polymerase chain reaction)?
To do PCR, the original DNA that one wishes to copy need not be pure or
abundant. It can be pure but it also can be a minute part of a mixture of
materials. So, PCR has found widespread and innumerable uses -- to diagnose
genetic diseases, do DNA fingerprinting, find bacteria and viruses, study human
evolution, clone the DNA of an Egyptian mummy, establish paternity or biological
relationships, etc.. Accordingly, PCR has become an essential tool for
biologists, DNA forensics labs, and many other laboratories that study genetic
material.
Next: How was PCR (polymerase chain reaction) discovered? »
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