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Bacteriophage T4 DNA polymerase is a DNA polymerase associated with bacteriophage T4. It extends an existing paired strand end in the $5^{\prime }\to 3^{\prime }$ direction using a DNA template and deoxynucleoside triphosphates. A primer supplies the extendable end. The enzyme also has $3^{\prime }\to 5^{\prime }$ exonuclease activity, supporting DNA polymerase proofreading; the characterized enzyme lacks a $5^{\prime }\to 3^{\prime }$ exonuclease function.^[ref]

Extension and removal are competing chemical outcomes, not contradictory strand directions. A simplified kinetic example assigns rates $k_{\mathrm{ext}}$ and $k_{\mathrm{exo}}$ to two possible exits from one enzyme–strand-end state. If both exits have exponential waiting times, the probability that extension happens first isStipulated rates $100$ and $1{\mathrm{s}}^{-1}$ give $P_{\mathrm{ext}}\approx 0.990$; rates $0.1$ and $10{\mathrm{s}}^{-1}$ give approximately $0.0099$. These illustrative numbers show how slowing extension relative to removal can favor proofreading. They are not measured constants for this enzyme or a complete model of its active-site transitions.

The same protein also supplied a historical binding target. In their 1990 primary study, C. Tuerk and L. Gold selected RNA sequences that bound T4 DNA polymerase, finding distinct selected sequences with equivalent binding constants.^[ref] Measuring binding to a protein and measuring its catalytic DNA synthesis answer different experimental questions. The selected RNA is an aptamer; it is not itself the polymerase.

A primer is an existing nucleic-acid strand that supplies an extendable end for template-directed synthesis. In DNA replication, a DNA polymerase extends a suitably paired $3^{\prime }$ end rather than starting an arbitrary new strand from two free nucleotides. The primer-template junction therefore identifies both an initiation site and an orientation. The primary characterization of bacteriophage T4 DNA polymerase explicitly requires a template and primer for synthesis.^[ref]

For a short sequence example, align template $3^{\prime }\text{-}\mathrm{TACGGA}\text{-}{5}^{\prime }$ with primer $5^{\prime }\text{-}\mathrm{ATG}\text{-}{3}^{\prime }$. The primer pairs with the first three template bases. Extension adds C, C and T to its $3^{\prime }$ end, producing $5^{\prime }\text{-}\mathrm{ATGCCT}\text{-}{3}^{\prime }$. The primer is part of the resulting strand; it is not merely a location marker that disappears when synthesis starts.

The distinction between template and primer is functional. The template specifies the next complementary base; the primer supplies the strand end being extended. A primer may be DNA or RNA in an appropriate biological or experimental system, but suitability depends on the enzyme and pairing conditions, not simply on calling an oligonucleotide a primer.

In an idealized polymerase chain reaction count, two correctly oriented primers permit repeated copying of a bounded target. If each cycle doubles the counted target molecules with no loss, $N_n=N_0{2}^n$; ten cycles from one target give $1024$ copies. This is a bookkeeping model, not a measured amplification yield. Failed priming, incomplete extension and competing products violate its assumptions. The original amplification study combined copying with a subsequent restriction-site assay, illustrating that making more material and identifying its sequence are separate experimental tasks.^[ref]