Genetic engineering requires the combination of a number of different molecular genetics techniques. PCR is one method that allows the cloning and studying of specific sequences of DNA. Here are some of the discoveries and tools that make PCR possible.
1. Thermophilic Organisms
The process of PCR involves repeated heating and cooling of the PCR mixture, during which time double-stranded DNA melts and rehybridizes. The process of elongation (building new chains of DNA from templates and primers requires polymerase enzymes that can withstand the heating and cooling cycles. The original thermostable polymerase was called Taq, isolated from a thermophilic bacterium called Thermophilus aquaticus, but now there are others, like Pfu (from Pyrococcus furiosus) which has proof-reading capabilities, Pwo, Tfl, Tli, and Tma, each from a different microorganism, and with it's own unique properties.
2. Gel Electrophoresis
When the many cycles of PCR are complete, the PCR tube contains a mixture of primers, templates and partially elongated DNA strands. In order to purify the cloned gene of interest, there has to be a method for separating DNA pieces of differing lengths. This method is agarose gel electrophoresis, made possible by the discovery that nucleotide fragments can be separated by moving them through a porous material (agarose) within an electric field and then visualized using fluorescent dyes.
3. DNA Sequencing
Without the ability to sequence DNA, we would not be able to create primers. Primers are the short pieces of DNA that complement each end of the gene you want to sequence. Therefore, an essential requirement of PCR is that you know at least a partial sequence of the gene of interest, or DNA sequences outside the gene at either end.
4. Nucleotide Synthesis
Hand-in-hand with the necessity of being able to sequence DNA, is the ability to synthesize short fragments of DNA (approximately 10-30 base pairs long) to make the primers used in a PCR reaction. We've had the ability to sequence DNA for many years. In fact, the first whole genome sequenced was in 1977. However, it has taken some improvements to the process, and automation (developed in the late 80s - early 90s) to make it fast and inexpensive enough for everyday use.