AP Biology pd. 2
March 16, 2004
PCR- Polymerase chain reaction is a method used to quickly and selectively amplify pieces of DNA instead of cloning the DNA. PCR is used when the source of DNA is insufficient or tainted. The initial DNA is incubated in a test tube with a heat-resistant type of DNA polymerase, a supply of all 4 nucleotides and primers. The primers are complementary to the ends of the targeted DNA so that they determine the particular segment of DNA that is to be amplified.
The solution is briefly heated to separate the DNA strands, then cooled so that the primers will hydrogen-bond. DNA polymerase now comes in handy; it adds nucleotides to the 3' end of the each primer. Each such cycle takes about 5 minutes and at the end of each cycle, the original DNA sequence has been doubled. The solution is reheated, starting the next cycle of strand separation, primer binding and DNA synthesis.
Since the primers are complementary to sequences bracketing the targeted sequence, they determine the DNA sequence that is amplified. PCR can be used to amplify a specific gene before additional cloning in cells. The PCR amplifies the gene, making it readily available and easy to locate, so that later on it is easier to find a clone carrying that gene.
PCR is so specific and powerful that its initial solution does not even have to be pure DNA. PCR can make do with DNA that is in poor condition or is scarce. PCR has had a major impact on understanding genetics. PCR has been used to quickly and selectively amplify DNA from various sources: 40, 000-year-old woolly mammoth; blood, tissue or semen found at the scenes of violent crimes; single embryonic cells for rapid prenatal diagnoses of genetic disorders; and viral genes from cells infected with difficult-to-detect viruses such as HIV.
Gel electrophoresis makes it possible to separate and isolate DNA restriction fragments of different...