The Polymerase Chain Reaction

PCR requires a template molecule of the DNA or RNA you want to copy and two primer molecules to get the copying process started. The primers are short chains of the four different base pairs that make up any strand of genetic material.

There are three basic steps in PCR. First, the target genetic material must be denatured. That is, the strands of its helix must be unwound and separated by heating to 90-96°C. The second step is hybridisation or annealing, in which the primers bind to their complementary bases on the now single-stranded DNA. The third is DNA synthesis by a polymerase. Starting from the primer, the polymerase can read a template strand and match it with complementary nucleotides very quickly. The result is two new helixes in place of the first, each composed of one of the original strands plus its newly assembled complementary strand.

To get more of the DNA you want, just repeat the process, beginning by denaturing the DNA you've already made. The amount will double every time. With the cycle of rapid heating and cooling controlled automatically, nature-aided by scientist-supplied primers, polymerase, nucleotides, and chemical reagents-does the rest. Each cycle takes only 1-3 minutes, so repeating the process for just 45 minutes can generate millions of copies of a specific DNA strand. Once the primers have been characterised and obtained, PCR can do in a week work that used to take a year. Several types of PCR machines have yet been designed. Mullis received a 1993 Nobel Prize for his invention.