A glossary of PCR terms

PCR vocabulary is part of the plan. If amplicon, primer-dimer, hot-start, Tm, Ta, plateau, master mix, NTC, inhibition, fidelity and multiplex are fuzzy, the programme you type will be fuzzy too. This protocol overview puts those words in the order you need them while you plan a run. The physical cycle they describe is in how polymerase chain reaction works. Use them before you open a tube, not as labels stuck on afterwards.

Enzymes and mixes are specified by these words. Start from the molecular biology catalogue and put the same words in the quote request.

Plan the amplicon before the temperatures

An amplicon is the DNA copied between the two primers, the molecule you expect to see, clone or sequence. Name its length in base pairs from a stated reference sequence. That length sets the extension hold for the enzyme class you will use. For a classical Taq-like polymerase, laboratories often plan on the order of one minute per kilobase around 72 Celsius. A high-fidelity enzyme may be faster or slower. The amplicon length without the enzyme class is an incomplete instruction.

While you are looking at the primers, look for primer-dimer. Primer-dimer is a short product made when primers anneal to each other and the polymerase extends them. It is usually small and fuzzy on a gel, and it can appear in a tube that never received template. It competes for enzyme and nucleotides. A pair that dimerises strongly will keep doing so after you have given the reaction a fashionable annealing temperature. Check candidate primers for complementarity, and check what else they might bind with a public tool such as NCBI Primer-BLAST.

Tm is a calculation, Ta is a setting

Tm, the melting temperature, is the temperature at which half of a primer-template duplex is dissociated under a stated salt condition. Calculators disagree because the salt, the magnesium and the nearest-neighbour parameters differ. A calculated Tm is a start. Write down which method you used so a later reader can repeat the arithmetic.

Ta, the annealing temperature, is the number in the cycler programme during the step where primers are meant to bind. It is usually planned a few degrees below the less stable primer's Tm, then adjusted because the real buffer is not the calculator's buffer. Additives and magnesium move the window. A gradient, or three tubes at neighbouring temperatures, finds the local Ta. Copying a Ta from a paper that used a different master mix is how specific reactions become smears.

Denaturation and extension have their own set-points. They are not Ta. Keep the three words apart in the written programme.

Hot-start, fidelity and the enzyme line

Hot-start is a block on the polymerase that heat later removes. The block may be an antibody, a chemical modification, or an aptamer. Its job is to reduce extension during setup, while tubes sit at room temperature and primers find partial matches. It does not repair a bad primer pair, and it often asks for a longer initial heat step than a non-hot-start enzyme. If your plan says hot-start, the programme must include the activation that enzyme requires. Follow the card.

Fidelity is the enzyme's tendency to insert the correct nucleotide, usually discussed as a class: proofreading or not. Proofreading enzymes carry a 3-prime to 5-prime exonuclease. Use that class when the amplicon will be cloned or sequenced as a reference. A presence assay that only needs a band can use a Taq-like enzyme and accept a higher error rate. Do not invent a numerical error rate for a bottle you have not read. The manufacturer states a fidelity claim for that preparation. Your plan either needs that claim or it does not.

Write both words on the same line of the plan: hot-start or not, proofreading or not. A proofreading enzyme may or may not be hot-start. Assuming one implies the other skips the activation step.

Master mix, NTC, inhibition, plateau

A master mix is the shared portion of the reaction, combined once and aliquoted so that tubes differ mainly by template. It typically holds polymerase, buffer, magnesium, dNTPs and, when the design allows, the primers. A dye for direct gel loading may be included. The point is fewer pipetting differences. The risk is that one contamination event reaches every tube. That is why the mix is made where finished amplicons are not handled.

NTC is the no-template control: master mix with water or buffer where the sample would have been. It asks whether the reagents and the handling made the amplicon without the DNA you claim to be testing. Plan its position before the samples, close it before template is open, and read it first. A full-length band in the NTC stops the run. A short primer-dimer in the NTC is a primer finding, and you should name it as such.

Inhibition is the failure in which the target is present and the enzyme still does not copy it, because something in the sample interferes. Heme, humic acids, leftover ethanol, chaotropic salts and too much DNA are familiar causes. The planning word means you decided, before the run, how you will tell inhibition from a true negative: a dilution of the sample, or a spike of a known target into the matrix. A plan that says "negative" with no inhibition check is unfinished whenever the matrix is dirty.

Plateau is the late phase in which product stops doubling. Reagents or active enzyme are limiting, or product strands anneal to each other and leave the primers short of template. Endpoint brightness lives here. Two samples can reach similar band intensities from different starting amounts. If your decision is presence and size, plateau is acceptable and you still keep a marker and an NTC. If your decision is a quantity, plateau is the reason to leave endpoint PCR. Cycle number is part of this choice. Adding cycles "to be sure" drives the reaction further into plateau and grows side products.

Multiplex is a different plan, not an extra primer

Multiplex means more than one primer pair in the same tube, aimed at more than one amplicon. Primer interactions multiply. The Ta becomes a compromise across pairs. Magnesium that suited one pair can liberate dimer from another. Products need sizes, or probes, that you can tell apart. A multiplex plan states every amplicon length, every pair, and the control that shows a blank for each product. The multiplex PCR enquiry reference is a place to ask that design question.

Where the words get swapped

Calling Ta the Tm skips the local window. Calling a plateau band a quantity invents a fold change from a saturated gel. Calling hot-start a cure for primer-dimer delays a new oligo design. Calling every blank an NTC hides the extraction negative, which saw the sample path. Calling a two-target tube ordinary PCR skips the compromise Ta. Read the plan line back against the definition it claims to use.

dNTPs chelate magnesium, so changing one without the other changes activity and specificity. A common classical range is about 1.5 to 2.5 millimolar magnesium and about 200 micromolar of each dNTP. Follow the mix you have.

Safety and the record

The hazards stay physical: a hot block, ultraviolet light at a gel, stains, and any infectious template your institution has not cleared. Biosafety containment is an institutional decision. A complete vocabulary clarifies the research sentence. It leaves that sentence non-diagnostic.

In a shared laboratory the written programme is what the next shift can rerun. Include amplicon length, enzyme class, hot-start activation, Ta and how you chose it, cycle count, NTC position, and what plateau allows you to claim.

What the enquiry should contain

Repeat the plan in miniature. Template matrix, amplicon length, endpoint or real-time, hot-start, fidelity class, inhibitor-rich sample if that is true, and the control set including the NTC. If the plan is multiplex, say how many products and how you will tell them apart. The nucleic acid analysis pathway is the wider path. Ask whether a quotation is possible for the enzyme class you named. A family name without fidelity and hot-start is not yet a specification.