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Graphical abstract, Highlights • Real-time study of DNA polymerase fingers conformations during DNA synthesis. • DNA polymerase adopts predominantly a fingers-closed conformation during synthesis. • The post-chemistry rate-limiting step occurs in the fingers-closed conformation. • The 3′-OH group substantially affects the kinetics of fingers conformational changes. • Approach well suited for multi-step reactions driven by proteins on nucleic acids., DNA polymerases maintain genomic integrity by copying DNA with high fidelity, part of which relies on the polymerase fingers opening-closing transition, a series of conformational changes during the DNA synthesis reaction cycle. Fingers opening and closing has been challenging to study, mainly due to the need to synchronise molecular ensembles. We previously studied fingers opening-closing on single polymerase-DNA complexes using single-molecule FRET; however, our work was limited to pre-chemistry reaction steps. Here, we advance our analysis to extensible substrates, and observe DNA polymerase (Pol) conformational changes across the entire DNA polymerisation reaction in real-time, gaining direct access to an elusive post-chemistry step rate-limiting for DNA synthesis. Our results showed that Pol adopts the fingers-closed conformation during polymerisation, and that the post-chemistry rate-limiting step occurs in the fingers-closed conformation. We found that fingers-opening in the Pol-DNA binary complex in the absence of polymerisation is slow (∼5.3 s−1), and comparable to the rate of fingers-opening after polymerisation (3.4 s−1); this indicates that the fingers-opening step itself could be largely responsible for the slow post-chemistry step, with the residual rate potentially accounted for by pyrophosphase release. We also observed that DNA chain-termination of the 3′ end of the primer increases substantially the rate of fingers-opening in the Pol-DNA binary complex (5.3 → 29 s−1), demonstrating that the 3′-OH residue is important for the kinetics of fingers conformational changes. Our observations offer mechanistic insight and tools to offer mechanistic insight for all nucleic acid polymerases.