What is missing in ddNTPs that causes chain termination?
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.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.