sentences:  The base pairing and chain formation reactions, which form the daughter helix, are catalyzed by DNA polymerases.These enzymes move along single-stranded DNA and allow for the extension of the nascent DNA strand by "reading" the template strand and allowing for incorporation of the proper purine nucleobases, adenine and guanine, and pyrimidine nucleobases, thymine and cytosine.Activated free deoxyribonucleotides exist in the cell as deoxyribonucleotide triphosphates (dNTPs).