PCR Machines

2270 & 9970 GenAmp System from Applied Biosystems International Inc, California, USA

Polymerase chain reaction (PCR) is a method that allows exponential amplification of short DNA sequences. It involves the use of 2 primers, each about 20 nucleotides in length that are complementary to a defined sequence on each of the two strands of the DNA to be amplified. First, 1 double stranded is denatured to 2 single strand DNAs. The primers bind to the single strand DNAs and the DNA polymerase makes a complementary strand to convert the single strand DNA to double strand DNA. Now 1 double strand DNA has becomes 2. This completes one cycle. In the next cycle, the 2 double strand DNAs are denatured to 4 single strand DNAs and all the 4 of them are converted to double strand DNAs. Likewise, in 30 cycles that takes about 2 hours, 1073741824 (=230) double strand DNA molecules can be from single DNA molecules! It should be noted that even the fast growing E.coli bacterium takes about 20 minutes to make one copy its DNA. 

For doing PCR, the test tube containing the reaction components are kept at 95C for 1 min for denaturation of the template DNA, at 55C for 1 min for annealing of the primers, and at 72C for 2 min for new strand synthesis. Originally three water baths were used for this purpose and test tubes were transferred between the water baths manually. Also fresh DNA polymerase should be added in each cycle after the denaturation step (95C) to prevent its denaturation. These were time consuming, laborious, often frustrating steps in PCR. This led to the fabrication of PCR machine which is nothing but a thermal block, temperature of which can be increased and decreased rapidly using peltier technology. 

PCR machine eliminated the need for using three water baths and manual transfer of tubes between them. Discovery of the thermostable Taq DNA polymerase obviated the need for adding fresh DNA polymerase in each cycle. These two achievements made PCR a really loving technology and brought Nobel Prize for its inventor, Kary Mullis in 1993 - that is 10 years after he invented it. Today PCR and PCR machines are ubiquitous in any good molecular biology lab.

Click here for PCR animation and to enjoy a PCR song