In the chain-terminator technique (Fig. 3-19), the DNA to be sequenced is incubated with DNA poly- merase I, a suitable primer, and the four dNTP substrates (reactants in enzy- matic reactions) for the polymerization reaction. The key component of the reaction mixture is a small amount of a 2ⴕ,3ⴕ-dideoxynucleoside triphos- phate (ddNTP), which lacks the 3¿-OH group of deoxynucleotides.When the dideoxy analog is incorporated into the growing polynucleotide in place of the corresponding normal nucleotide, chain growth is terminated because addition of the next nucleotide re- quires a free 3¿-OH.