An exonuclease removes nucleotides from an end of a nucleic acid, in contrast to an endonuclease that cuts internal bonds. Direction, strand preference and substrate structure vary between enzymes. A proofreading exonuclease associated with some DNA polymerases can remove an incorrectly added terminal nucleotide before synthesis resumes. Consequently, exonucleolytic removal and polymerization can contribute to accuracy through different catalytic activities. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 5, DNA Replication Mechanisms: proofreading exonucleases
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