To each reaction is added only one of the four dideoxynucleotides (ddATP, ddGTP, ddCTP, or ddTTP), while the other added nucleotides are ordinary ones.
En cada reacción engádese só un dos catro didesoxinucleótidos (ddATP, ddGTP, ddCTP ou ddTTP), mentres que os outros nucleótidos engadidos son os normais.