This DNA sequencing technique, also known as "chain termination sequencing" or "deoxy sequencing", was developed by the biochemist Frederick SANGER in 1977 and involved two steps:

1. A specific PCR: it differs from a conventional PCR using a single primer (instead of two) and by the incorporation of chemically modified nucleotides blocking the elongation of the fragment, the dideoxynucleotides (ddNTPs). The presence of a single primer prevents the amplification of the fragment to be sequenced. The ddNTPs, on the other hand, allow to obtain fragments of different lengths. The concentrations of ddNTPs used make it possible to statistically synthesize all fragment lengths. In the basic method, 4 reactions were performed simultaneously, each including a specific ddNTP (ddATP, ddTTP, ddCTP, ddGTP). Nowadays, each of the 4 ddNTPs carries a specific fluorochrome, which allows all DNA fragments to be labelled at their ends during a single reaction.
2. Fragment separation: in the original method, the products of each reaction were separated by electrophoresis trough distinct agarose gel tracks. As the fragments were separated according to their length, it was sufficient to "read" the bands on the gel to determine the sequence. Nowadays, the DNA fragments obtained are subjected to capillary electrophoresis during which the fluorescent signal emitted by each labelled ddNTP is analyzed by computer. Data processing allows to determine the identity of the nucleotide and to reconstruct/reconstitute the sequence of the original DNA template.