The sequencing method is known as the dideoxy chain termination method. The method is based on the use of chain terminators, the dideoxynucleotides (ddNTPs). The ddNTPSs differ from the deoxynucleotides by the lack of a free 3′ OH group on the five-carbon sugar.

If a ddNTP is added to a growing DNA strand, the chain cannot be extended any further because the free 3′ OH group needed to add another nucleotide is not available. By using a predetermined ratio of deoxyribonucleotides to dideoxynucleotides, it is possible to generate DNA fragments of different sizes.

In Frederick Sanger’s dideoxy chain termination method, dye-labelled dideoxynucleotides are used to generate DNA fragments that terminate at different points.

The DNA is separated by capillary electrophoresis (not defined) on the basis of size, and from the order of fragments formed, the DNA sequence can be read. The DNA sequence readout is shown on an electropherogram (not defined) that is generated by a laser scanner.