What dNTPs magnesium and polymerase each do

dNTPs, magnesium and the polymerase are the three components people turn when a PCR misbehaves, and they are not three knobs on one dial. Nucleotides are the substrate. Magnesium is the cofactor the enzyme and the duplex share, and nucleotides bind it. The polymerase is the catalyst, and its proofreading behaviour is a cloning decision rather than a way to make a lane brighter. Change one of them at a time or the gel will not tell you which change mattered. The cycle they sit inside is explained in how polymerase chain reaction works.

What each one is for

The four dNTPs are the substrates, one for each base. The polymerase adds the nucleoside monophosphate and releases pyrophosphate. A missing nucleotide stops the chain at the first base that needed it. A strong imbalance among the four raises misincorporation, which matters if you will clone and which a gel will not show.

A common classical planning concentration is about 200 micromolar of each dNTP. Long amplicons spend more per copy, so a level that suits a short product can starve a multi-kilobase one. Far above that band, the extra phosphate chelates magnesium and some enzymes become less accurate. Follow the mix. Do not add a second dNTP aliquot to a complete master mix because another enzyme's protocol listed nucleotides separately.

Magnesium is required at the polymerase active site, and it stabilises the primer-template duplex. Free magnesium is the fraction not bound to dNTPs, EDTA, phosphate or other chelators that arrived with the template. A common classical planning range for total magnesium is about 1.5 to 2.5 millimolar. That number is total, not free. Four dNTPs at 200 micromolar each are 0.8 millimolar nucleotide, and each can bind a magnesium ion, so a large share of a 1.5 millimolar total is already spoken for. This is why copying either number without the other is incoherent. The card for a complete mix has already done the arithmetic. Adding the paper's magnesium on top double-counts it.

The polymerase chooses the chemistry of extension. A Taq-like enzyme typically lacks a 3-prime to 5-prime proofreading exonuclease, often leaves a single A overhang, and is a fair choice when you need presence and size. A proofreading enzyme carries that exonuclease, removes many mismatched bases, and often leaves blunt ends. Choose it when the product will be cloned or when a substituted base would change the biological claim. Fidelity is errors per base incorporated. No gel stain reports it. A brighter Taq lane is not a more faithful sequence. How that sequence is checked after cloning is the subject of plasmid cloning from insert to colony.

Hot-start is not a fourth member of this trio. It is a block on when the enzyme may work. A hot-start Taq-like enzyme is still Taq-like. A proofreading hot-start enzyme is still a proofreading enzyme. Do not buy a hot-start label and believe you have bought fidelity.

One variable, including the coupling you must admit

The honest experiment changes one named component and keeps a tube of the previous mix on the same template, plus a no-template control. Read whether the expected size appears, and whether extra bands appear. Do not pick the winner by which band photographs hotter.

dNTPs and magnesium refuse to be fully independent. If you raise dNTPs and hold total magnesium still, free magnesium falls. The gel may look less specific or more specific, or it may go blank, and you cannot write "nucleotides did this" as if magnesium had stayed constant. The practical rule is: titrate magnesium at a fixed dNTP concentration when specificity is the question. If you truly must change dNTPs, for a long amplicon that is starving, re-check magnesium afterwards rather than comparing the new nucleotide level to the old tube as a pure nucleotide result. Say the coupling in the notebook.

Changing the polymerase changes buffer, hot-start class, speed and fidelity together, because you rarely have the same buffer in the second tube. Minimise the confound by using each enzyme in the buffer the card pairs with it, and do not also "improve" magnesium in the same afternoon. If both enzymes fail, the primers or the template are the more likely common cause. If only the proofreading enzyme fails, check its activation hold, its extension speed, and whether an additive in the old Taq recipe is forbidden on the new card.

Activity units are not a shared currency. Match the amount the open card recommends. A unit defined on activated salmon sperm DNA in one catalogue is not a unit defined in another.

A workflow that stays interpretable

If the mix is complete, the first reaction uses its stated dNTPs, magnesium and enzyme amount. A blank is not yet a reason to titrate all three. If extra bands appeared after magnesium was added, take that magnesium back before you buy a new polymerase. If a long amplicon stalls, look at extension time and processivity before you double the dNTPs. If the product must be cloned, switch to a proofreading class in its own buffer as a separate experiment, then sequence. The new gel only shows length.

EDTA in a resuspension buffer is a silent magnesium sink. A template dissolved in a high-EDTA storage buffer can blank a reaction that works on water-resuspended DNA. That is a chelator problem, not a weak polymerase. Dilute the template or change the storage buffer rather than pouring in more magnesium until something appears. More magnesium will also loosen specificity on every other tube you treat the same way.

What these components will not do

They will not make endpoint PCR quantitative. They will not clean a contaminated no-template control. They will not force a primer with a mismatched 3-prime end to extend. They will not tell you the sequence. A proofreading enzyme lowers the chance of polymerase error. It does not remove errors that were already in the template, and it does not remove the need to read the clone.

Comparing two enzymes by a single bright band, at different cycle counts, is not a fidelity test and not even a yield test. Match the cycles if you insist on comparing yield, and send the winner to sequencing if the claim needs the bases.

Safety, aliquots and heat

dNTP stocks and enzyme are ordinary molecular-biology reagents. Magnesium salts are not an excuse to ignore the template's biosafety level. Hot blocks still burn. This guide is research practice, not a clinical formulation.

Shared freezers that warm on a power cut punish master mixes. A mix that thawed and refroze has changed nucleotide stability and can change how much magnesium is effectively available if components have precipitated or been left open in humid air. Aliquot the mix when it first arrives, and do not return a thawed aliquot to the parent tube. A one-variable test the next morning is meaningless if the stock itself moved overnight. Label the aliquot with the enzyme class so a proofreading tube is not later judged by how bright it looks next to Taq.

What to put in an enquiry

Name the amplicon length, whether the product will be cloned, whether you need proofreading or a Taq-like enzyme, and whether the buffer must already contain magnesium. State the result of any one-variable test you already ran. Families of enzymes and nucleotides are grouped in the molecular biology catalogue. Send the requirement with the quote request. A family name is not a formulation. If the template is an extract that may carry EDTA or inhibitors, say so via the nucleic acid analysis pathway. Several targets in one tube, each spending the same dNTP pool, are a question for the multiplex PCR enquiry reference. The long product feels a nucleotide shortage first.