<p>In genetics and biochemistry, <strong>sequencing</strong> means to determine the primary structure (or primary sequence) of an unbranched biopolymer. Sequencing results in a symbolic linear depiction known as a <strong>sequence</strong> which succinctly summarizes much of the atomic-level structure of the sequenced molecule.</p>

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<h2><span class="mw-headline">DNA sequencing</span></h2>

<h3><span class="mw-headline">Sanger sequencing</span></h3>

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Part of a radioactively labelled sequencing gel</div>

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<p>In chain terminator sequencing (Sanger sequencing), extension is initiated at a specific site on the template DNA by using a short oligonucleotide 'primer' complementary to the template at that region. The oligonucleotide primer is extended using a DNA polymerase, an enzyme that replicates DNA. Included with the primer and DNA polymerase are the four deoxynucleotide bases (DNA building blocks), along with a low concentration of a chain terminating nucleotide (most commonly a <strong>di-</strong>deoxynucleotide). Limited incorporation of the chain terminating nucleotide by the DNA polymerase results in a series of related DNA fragments that are terminated only at positions where that particular nucleotide is used. The fragments are then size-separated by electrophoresis in a slab polyacrylamide gel, or more commonly now, in a narrow glass tube (capillary) filled with a viscous polymer.</p>

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View of the start of an example dye-terminator read (click to expand)</div>

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