The chain-terminator method (also called the dideoxy method) uses an E. coli enzyme to make complementary copies of the single-stranded DNA being sequenced. The en- zyme is a fragment of DNA polymerase I, one of the enzymes that partic- ipates in replication of bacterial DNA (Section 25-2A). Using the single DNA strand as a template, DNA polymerase I assembles the four deoxynu- cleoside triphosphates (dNTPs), dATP, dCTP, dGTP, and dTTP, into a com- plementary polynucleotide chain that it elongates in the 5¿S3¿ direction (Fig. 3-18). DNA polymerase I can sequentially add deoxynucleotides only to the 3¿ end of a polynucleotide. Hence, replication is initiated in the presence of a short polynucleotide (a primer) that is complementary to the 3¿ end of the template DNA and thus becomes the 5¿ end of the new strand. The primer base-pairs with the template strand, and nucleotides are sequentially added to the 3¿ end of the primer. If the DNA being sequenced is a restriction frag- ment, as it usually is, it begins and ends with a restriction site.