Practice question

Dideoxynucleotides are analogs of deoxynucleotides that lack hydroxyl groups at both the 2' and 3' positions of the ribose sugar. Deoxynucleotides lack only the 2' OH, retaining 3' OH essential for chain elongation. DNA polymerase catalyzes nucleophilic attack of the primer 3' OH on the alpha-phosphate of incoming nucleotide. When a ddNTP is incorporated, no 3' OH remains to attack the next nucleotide, so phosphodiester bond formation cannot continue. Consequently, chain growth aborts. Missing base or 5' OH does not explain termination; absence of 3' OH is critical.