Deoxyadenosine triphosphate ($\mathrm{dATP}$)

Deoxyadenosine triphosphate, usually abbreviated dATP, is a nucleotide containing adenine, a deoxyribose sugar and three phosphate groups. The “deoxy” distinguishes its sugar from the ribose in adenosine triphosphate. The usual molecular formula for the neutral acid is ${\mathrm{C}}_{10}{\mathrm{H}}_{16}{\mathrm{N}}_5{\mathrm{O}}_{12}{\mathrm{P}}_3$; its ionization state in solution depends on conditions.^[ref]

A DNA polymerase uses dATP as the substrate for adding an adenine-containing unit to a growing DNA strand opposite a template thymine. The strand's terminal $3^{\prime }$ hydroxyl attacks the incoming nucleotide's alpha phosphate; the other two phosphates leave as pyrophosphate. The incorporated unit is therefore a monophosphate residue within the chain, not a triphosphate hanging from the completed backbone. This distinction connects nucleotide chemistry with the directional $5^{\prime }\to 3^{\prime }$ growth of DNA.^[ref]

A polymerase assay can supply the template, primer, enzyme and nucleotide substrates, then compare product lengths with and without dATP. A failure to extend past a template position requiring adenine tests substrate dependence; it does not by itself determine every enzymatic step.