<P> Polymerase chain reaction (PCR) is a technique used in molecular biology to amplify a single copy or a few copies of a segment of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence . It is an easy, cheap, and reliable way to repeatedly replicate a focused segment of DNA, a concept which is applicable to numerous fields in modern biology and related sciences . </P> <P> Developed in 1983 by Kary Mullis, PCR is now a common and often indispensable technique used in clinical and research laboratories for a broad variety of applications . These include DNA cloning for sequencing, gene cloning and manipulation, gene mutagenesis; construction of DNA - based phylogenies, or functional analysis of genes; diagnosis and monitoring of hereditary diseases; amplification of ancient DNA; analysis of genetic fingerprints for DNA profiling (for example, in forensic science and parentage testing); and detection of pathogens in nucleic acid tests for the diagnosis of infectious diseases . In 1993, Mullis was awarded the Nobel Prize in Chemistry along with Michael Smith for his work on PCR . </P> <P> The vast majority of PCR methods rely on thermal cycling, which involves exposing the reactants to cycles of repeated heating and cooling, permitting different temperature - dependent reactions--specifically, DNA melting and enzyme - driven DNA replication--to quickly proceed many times in sequence . Primers (short DNA fragments) containing sequences complementary to the target region, along with a DNA polymerase, after which the method is named, enable selective and repeated amplification . As PCR progresses, the DNA generated is itself used as a template for replication, setting in motion a chain reaction in which the original DNA template is exponentially amplified . The simplicity of the basic principle underlying PCR means it can be extensively modified to perform a wide array of genetic manipulations . PCR is not generally considered to be a recombinant DNA method, as it does not involve cutting and pasting DNA, only amplification of existing sequences . </P> <P> Almost all PCR applications employ a heat - stable DNA polymerase, such as Taq polymerase, an enzyme originally isolated from the thermophilic bacterium Thermus aquaticus . This DNA polymerase enzymatically assembles a new DNA strand from free nucleotides, the building blocks of DNA, by using single - stranded DNA as a template and DNA oligonucleotides (the primers mentioned above) to initiate DNA synthesis . </P>

How do primers target a particular dna sequence
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