The enzymatic polymerization of DNA and RNA is the basis for genetic inheritance for all living organisms. It is catalyzed by the DNA/RNA polymerase (Pol) superfamily. Here, bioinformatics analysis reveals that the incoming nucleotide substrate always forms an H-bond between its 3'-OH and beta-phosphate moieties upon formation of the Michaelis complex. This previously unrecognized H-bond implies a novel self-activated mechanism (SAM), which synergistically connects the in situ nudeophile formation with subsequent nucleotide addition and, importantly, nucleic acid translocation. Thus, SAM allows an elegant and efficient closed-loop sequence of chemical and physical steps for Pol catalysis. This is markedly different from previous mechanistic hypotheses. Our proposed mechanism is corroborated via ab initio QM/MM simulations on a specific Pol, the human DNA polymerase-eta, an enzyme involved in repairing damaged DNA. The structural conservation of DNA and RNA Pols supports the possible extension of SAM to Pol enzymes from the three domains of life.