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An amplicon is a nucleic acid product of amplification. In ordinary PCR amplification without added primer tails, its boundaries eventually correspond to the outer ends of the two PCR primer binding sites. First-cycle extensions on long starting templates can run beyond the opposite site; exact-length single strands appear in cycle 2 and exact-length duplex products in cycle 3 under complete extension. This explains why the bounded product becomes dominant even though the initial extensions need not have its length.

PCR controls are scientific controls that separate different explanations for a readout. A no-template reaction contains the PCR primers and copying reagents but replaces the added template with water or sample buffer. Amplification there can indicate carried-over DNA or a primer dimer; a clean result does not exclude contamination confined to one sample.^[ref]

Different comparisons answer different questions. A known amplifiable template tests whether the PCR primers, DNA polymerase and thermal cycler can produce the expected amplicon. An extraction blank follows sample preparation without adding the sampled material. A separate amplifiable interval in the sample can test whether recovered DNA supports copying, although success there does not guarantee success at the target. A length standard calibrates gel electrophoresis; it is not a control for the copying reaction. These comparisons are an analytical design extension to the teaching example below, not a claim that its student protocol includes every comparison.

A PCR primer is a short DNA strand that supplies a paired $3^{\prime }$ end for a DNA polymerase. Its nucleotide sequence determines where base pairing can initiate copying. Matching a PCR primer at one site is insufficient: both PCR primers must support opposed extension across a copyable interval for sustained selective PCR amplification.

PCR specificity concerns whether the accumulated amplicon comes from the intended interval. Binding temperature, PCR primer nucleotide sequences and their possible alternative binding sites matter. Too permissive binding can support competing products; excessively restrictive binding can prevent the intended product. A band of the expected length is useful evidence, but unrelated nucleotide sequences can have the same length. DNA sequencing tests identity more directly, subject to mixtures and copying errors.^[ref]

A PCR thermal cycle separates three jobs. During PCR denaturation, heating separates the paired DNA strands without intentionally cutting their phosphodiester bonds. During PCR annealing, cooling permits base pairing between PCR primers and templates. During PCR extension, the chosen interval supports DNA polymerase synthesis from the paired $3^{\prime }$ ends. Each new nucleotide is supplied as a deoxynucleoside triphosphate; its incorporation lengthens the strand. The DNA polymerase is a catalyst, while these nucleotides are consumed substrates.^[ref]

Taq polymerase withstands repeated high-temperature strand separation better than a heat-sensitive DNA polymerase. Its extension activity still depends on temperature, buffer solution and magnesium ions. Heat resistance does not mean constant activity at every stage: the separation temperature serves the DNA, whereas the extension temperature serves the copying enzyme. As one institutional example, Addgene's Basic PCR Program specifies $94^{\circ }\mathrm{C}$ separation, $55^{\circ }\mathrm{C}$ primer binding and $72^{\circ }\mathrm{C}$ extension. These are example settings, not universal properties of all PCR primers or DNA polymerases.^[ref]

The polymerase chain reaction amplifies a selected DNA region using repeated copying steps. Amplified material can be examined by DNA sequencing; appropriate template controls help distinguish a new rearrangement from material already present in the starting preparation.

The essential trick is to choose two PCR primers on opposite DNA strands with their extendible $3^{\prime }$ ends facing into the same interval. A DNA polymerase extends each PCR primer $5^{\prime }\to 3^{\prime }$; the resulting DNA can itself become a template. The amplicon is therefore selected by two opposed PCR primers, rather than by the DNA polymerase recognizing a named gene. This makes PCR amplification useful for inspecting one interval within a much larger genome. Here PCR means polymerase chain reaction; the statistical abbreviation PCR means principal component regression.^[ref]

For a deliberately tiny nucleotide sequence example, display the upper strand as $5^{\prime }\text{-}\mathrm{ACGT}\text{-}\mathrm{GGAATT}\text{-}\mathrm{CAGC}\text{-}{3}^{\prime }$. The left PCR primer is $5^{\prime }\text{-}\mathrm{ACGT}\text{-}{3}^{\prime }$ and binds the lower strand. The right PCR primer, written in its own $5^{\prime }$-to-$3^{\prime }$ direction, is $5^{\prime }\text{-}\mathrm{GCTG}\text{-}{3}^{\prime }$: the reverse complement of CAGC. It binds the upper strand and extends leftward in the drawing. With flanks outside this interval, the bounded amplicon contains $4+6+4=14$ base pairs, including both PCR primer sequences. These four-nucleotide PCR primers illustrate direction and counting only; they are not a proposed experimental design.

A primer dimer is an amplification artifact in which PCR primers provide extendible partners for one another. Its accumulation consumes substrates and may produce a signal even without the intended template. It is not evidence that the intended amplicon was present.^[ref]

The DNA Learning Center's Detection of an Alu Insertion Polymorphism by PCR by DNA Learning Center, Cold Spring Harbor Laboratory supplies a concrete length-based example. It collects mouthwash biological cells, concentrates them with a microcentrifuge, heats them by Chelex extraction and transfers the DNA-containing supernatant. Its $25\mu \mathrm{L}$ reaction combines $2.5\mu \mathrm{L}$ of that preparation with primer mix and a reagent bead in a thermal cycler. Thirty cycles use $94$, $68$ and $72^{\circ }\mathrm{C}$, each for thirty seconds. An electrophoresis chamber separates products by agarose gel electrophoresis in a 2 percent gel; a UV transilluminator reveals stained DNA.^[ref]

Its PCR primers flank an insertion site at the PV92 locus. The student protocol predicts a 715-base pair product with the Alu element and a 415-base pair product without it. Two bands support a heterozygous genotype; one corresponding band supports a homozygous genotype only if amplification and detection of both alleles were adequate. These are the protocol's expected readouts, not newly collected observations.^[ref]

The instructor manual describes extra bands from nonspecific priming and a small diffuse primer dimer band. A missing target band therefore cannot on its own distinguish failed extraction, inhibited copying, poor binding or lack of the target. Nor does detection of the Alu element at this one genetic locus establish a complete genome, a unique personal identity or a clinical conclusion. The PCR controls help separate these possibilities; DNA sequencing can add sequence evidence to the size comparison.^[ref]