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The PCR method is extremely sensitive, requiring only a few DNA molecules in a single reaction for amplification across several orders of magnitude. Therefore, adequate measures to avoid contamination from any DNA present in the lab environment ([[bacteria]], [[virus]]es, or human sources) are required. Because products from previous PCR amplifications are a common source of contamination, many molecular biology labs have implemented procedures that involve dividing the lab into separate areas.<ref name="balin">{{cite journal |vauthors=Balin BJ, Gérard HC, Arking EJ, etal |title=Identification and localization of Chlamydia pneumoniae in the Alzheimer's brain |journal=Med. Microbiol. Immunol. |volume=187 |issue=1 |pages=23–42 |year=1998 |pmid=9749980 |doi= 10.1007/s004300050071|s2cid=25307947 |quote="Extreme care was taken in all assays to avoid cross-contamination of both nucleic acid samples to be analyzed and reaction mixtures; such measures included preparation of nucleic acids in a laboratory separate from those in which PCR or reverse transcription (RT)-PCR assays were set up and use of eight different biologic hoods, each in a different laboratory, for setting up reactions." <!--NOTE: This "quote" was a "note". Someone with access to the full text of the article should verify that it actually exists in the text. It is not in the abstract.--> }}</ref> One lab area is dedicated to preparation and handling of pre-PCR reagents and the setup of the PCR reaction, and another area to post-PCR processing, such as [[gel electrophoresis]] or PCR product purification. For the setup of PCR reactions, many [[standard operating procedure]]s involve using [[pipette]]s with [[filter tips]] and wearing fresh [[medical gloves|laboratory glove]]s, and in some cases a [[laminar flow cabinet]] with UV lamp as a work station (to destroy any [[extraneomultimer]] formation). PCR is routinely assessed against a [[negative control]] reaction that is set up identically to the experimental PCR, but without template DNA, and performed alongside the experimental PCR.
[[File:PCR_gel_electrophoresis.jpg|alt=A Graph of PCR Gel electrophoresis|right|thumb|PCR Gel electrophoresis]]
== Hairpins ==
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[[Taq polymerase]] lacks a [[Directionality (molecular biology)|3′ to 5′]] [[exonuclease|exonuclease activity]]. Thus, Taq has no error-[[Proofreading (biology)|proof-reading activity]], which consists of excision of any newly misincorporated nucleotide base from the nascent (i.e., extending) DNA strand that does not match with its opposite base in the complementary DNA strand. The lack in 3′ to 5′ proofreading of the Taq enzyme results in a high error rate (mutations per nucleotide per cycle) of approximately 1 in {{formatnum:10000}} bases, which affects the fidelity of the PCR, especially if errors occur early in the PCR with low amounts of starting material, causing accumulation of a large proportion of amplified DNA with incorrect sequence in the final product.<ref> {{cite journal |vauthors= Eckert KA, Kunkel TA |title=DNA polymerase fidelity and the polymerase chain reaction |journal=Genome Research |volume=1 |issue=1 |pages=17–24 |date=August 1991 |pmid=1842916 |doi= 10.1101/gr.1.1.17 |url= http://www.genome.org/cgi/pmidlookup?view=long&pmid=1842916 |doi-access= free }}</ref>
Several "high-fidelity" [[thermostable DNA
==Magnesium concentration==
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