Chain termination method:

An incubation combination is set up containing the single-stranded DNA template, DNA polymerase I, the primer and all four deoxyribonucleoside triphosphates (dTTP, dGTP, dCTP, dATP), one of that is radioactively labeled plus a single 2'3' dideoxyribonucleoside triphosphate analog, say ddGTP. Within this incubation the DNA polymerase starts copying template molecules through extending the bound primer. As the latest DNA strand is synthesized, every time which dGTP should be incorporated there is a chance which ddGTP will be incorporated instead. If this occurs, no further chain elongation can occur because dideoxy analogs lack the 3'-OH group required to make the next 3'5' phosphodiester bond. Therefore this particular chain stops at this point. In this first incubation combination, a huge population of templates is being copied and every new strand will stop randomly at positions where a G must be added to the latest synthesized strand. Therefore, for every G in the complementary sequence there will be some new DNA strands which have terminated at that point shown in the figure.

In reality, four incubation combination are set up, each containing the similar components except which each contains a variant dideoxy analog; one of ddTTP , ddCTP, ddATP or ddGTP. This produces four groups of chain-terminated fragments corresponding to the positions of C and T, A and G in the sequence. After the incubation, all four reaction combinations are electrophoresed in equal lanes of a polyacrylamide gel and then subjected to autoradiography. DNA sequence is determined simply through reading the band pattern on the autoradiogram from the bottom of the gel towards the top for instance reading the DNA sequence as it is synthesized from the primer. The sequence read off the gel is the sequence of the synthesized DNA strand and therefore is the complementary sequence to the real DNA template strand.

Figure:   DNA sequencing by the chain termination (Sanger) method.