The PCR reaction requires a pair of short single-stranded oligonucleotide primers—usually 15–25 base pairs in length—that are each specific to a short region of sequence onopposing strands of the double-stranded genomic DNA. These primers are used in combinationwith a DNA polymerase (usually Taq polymerase), dNTPs, and reagent buffers to amplify thetarget DNA between and including the two primer target sequences. The target DNA, typically100–2000 bp in length, is copied in an exponential fashion by cycling the reaction mixturethrough a range of temperatures. PCR can have a detection limit of femtograms of DNA andcan, under ideal conditions, detect a single copy of a gene. However, practically the detectionlimits are much higher, ranging from 10 to 104 copies of a gene depending on other reactionvariables. Variables that strongly affect the sensitivity of the PCR include the purity of thetarget DNA, the presence of inhibitors, the specificity of the primers to the target DNA, and theratio of nontarget to target DNA in the sample. Care must be taken in the sample preparationwhich usually includes a lysis step to release the DNA followed by a purification step to removePCR inhibitory compounds. PCR can also be prone to false positives without the adherenceof strict laboratory cleanliness procedures to ensure that the DNA previously amplified in thelaboratory does not contaminate subsequent reactions. Cross-reactivity of the oligonucleotideprimers to nontarget DNA can also be a problem due to the similarity of the DNA sequencebetween closely related strains.