= Sanger-Nicklen-Coulson sequencing experiment
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In 1977, <Frederick Sanger>, <S. Nicklen> and <Alan Coulson> tested chain termination on <Phi X 174> <DNA>. They isolated <restriction enzyme> fragments as primers, heated them with template to $100^{\circ}\mathrm{C}$ for three minutes and annealed at $67^{\circ}\mathrm{C}$ for thirty minutes. Four extension mixtures used the <Klenow fragment> of <DNA polymerase> and radioactive dATP. Their Figure 1 used ddGTP, ddATP, ddTTP and araCTP, an alternative terminating substrate; Figure 2 used all four dideoxy types. This distinction matters when relating the original experiments to an all-ddNTP teaching model.<DNA sequencing with chain-terminating inhibitors by Frederick Sanger and others>[Sanger, Nicklen and Coulson, DNA sequencing with chain-terminating inhibitors, PNAS 74 (1977), pp. 5463–5465, Sequencing Procedure and Figures 1–2]{ref}
Figure 1 used a 12% acrylamide gel at $40\,\mathrm{mA}$ for fourteen hours, read by <autoradiography>. Omitting the unlabeled dATP chase caused unwanted A stops across samples: a chemistry <scientific control>. Ordered bands supported local <nucleotide sequences>; comparisons could revise provisional assignments. Weak T bands, unresolved long runs and structure-dependent migration limited interpretation. The paper's observations are evidence from particular preparations, not a guarantee that every endpoint is resolved.<DNA sequencing with chain-terminating inhibitors by Frederick Sanger and others>[Sanger, Nicklen and Coulson (1977), p. 5464, chase-omission observation and Figure 1 caption; p. 5466, Results: weak T bands, unresolved runs, sequence comparisons and structure-dependent migration]{ref}
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