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Not all microorganisms can be grown in culture. In fact, microbiologists have figured out how to grow very few species in the lab, less than 1% of all the microbial species in the world.
PCR allows scientists to extract and analyze bits of microbial DNA from samples, meaning they don’t need to find and grow whole cells.
PCR is an essential element in DNA fingerprinting and in the sequencing of genes and entire genomes.
Basically, it’s like a technique to photocopy pieces of DNA. In a matter of a few hours, a single DNA sequence can be amplified to millions of copies.
PCR lets scientists work with samples containing even very small starting amounts of DNA. The technique makes use of the DNA repair enzyme polymerase. This enzyme, present in all living things, fixes breaks or mismatched nucleotides in the double-stranded DNA helix. These breaks or mismatches could cause genes to malfunction if left unfixed.
Polymerase uses the intact half of the DNA molecule as a template and attaches the right nucleotides, which circulate constantly in the cell, to the complementary nucleotide at the site of the break. (DNA consists of two strands of nucleotide bases, which are represented as A, G, C, and T. In the laws of DNA base-pairing, A joins with T and G with C.)
Not all polymerases are created equal, however. Many fall apart in high heat. PCR was developed in 1985 following the discovery of an unusual heat-loving bacterium called Thermus aquaticus in a hot spring in Yellowstone National Park. This bacterium’s polymerase, dubbed Taq, does its job of matching and attaching nucleotides even in the high heat generated by the successive “photocopying” cycles required during PCR. Taq made PCR possible.