From the wikipedia article [1] "PCR is used to amplify a specific region of a DNA strand (the DNA target). Most PCR methods typically amplify DNA fragments of u

From the wikipedia article [1] "PCR is used to amplify a specific region of a DNA strand (the DNA target). Most PCR methods typically amplify DNA fragments of up to ~10 kilo base pairs (kb), although some techniques allow for amplification of fragments up to 40 kb in size.[5]" What that means is this: you have a sample of some DNA or protein. Typically it might be only one copy or only a handful of copies. Nothing in bioscience works on tiny copies; you typically need thousands or millions, particularly for operations like dna sequencing. PCR allows you to amplify -- ie copy -- your sample by causing the DNA helix to split from heat, including a polymerase or enzyme that does the replication work, including spare dna bases, then cooling to cause the helixes to reform. It's essentially a doubling operation; you run k heating/splitting/replication/cooling cycles and at the end hopefully have (#{ starting copies }) * 2^k Doing the previous operation requires a pcr device which very accurately and evenly heats and cools a microtitre plate; the polymerase; something to dump all this into like very clean water; and raw dna base pairs. [1] http://en.wikipedia.org/wiki/Polymerase_chain_reaction http://en.wikipedia.org/wiki/Polymerase_chain_reaction