Answer :
Labeling either the DNA sequencing primers or the individual nucleotides with a radioactive or fluorescent tag enables this visualization. In automated sequencing, the ddNTPs are labeled with fluorescent dyes that are detected by a scanner. This makes it possible for all four reactions (dGTP, dATP, dCTP, and dTTP) to be run in one lane, so you can have huge numbers of reactions on one gel.