= PCR thermal cycle
{c}
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 $3'$ 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.<Principles of PCR by Thermo Fisher Scientific>[Principles of PCR, accompanying description: denaturation, annealing, primer-dependent extension and dNTP incorporation]{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>.<Polymerase Chain Reaction (PCR) by Addgene>[Basic PCR Program, steps 1–6; Materials List: Taq buffer with magnesium chloride]{ref}
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