An incubation  combination  is set up having  the single-stranded  DNA template, DNA polymerase  I, the primer and all 4 deoxyribonucleoside triphosphates (dTTP, dGTP, dCTP, dATP), one of that is radioactively  labeled plus a one 2'3' dideoxyribonucleoside triphosphate  analog, say ddGTP. Within this incubation the DNA polymerase   starts copying template molecules   by extending   the bound primer.  As the new DNA  strand  is synthesized,  every  time which dGTP should  be  incorporated   there  is  a  chance  which  ddGTP  will  be  incorporated instead. If this happens, no further chain elongation  can occur because dideoxy analogs  lack  the  3'-OH  group  needed  to  make  the  next  3'5'  phosphodiester bond.  Thus this particular chain stops at this point.  In this first incubation combination, a huge population of templates is being copied and each new strand will stop randomly at positions where a G must be added to the latest synthesized strand.  Thus, for every G in the complementary sequence there will be some new DNA strands which have terminated at that point shown in the figure.

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