FAQ: Can Exonuclease I be used to blunt DNA?

No. Short ends are not a substrate. Use DNA Polymerase I, Large (Klenow) Fragment (NEB# M0210) to fill in 5' overhangs (also called 3' recessed ends) and chew back 3' overhangs or use Mung Bean Nuclease (NEB# M0250) to chew back 3' or 5' overhangs. Note: 3' overhangs can not be filled in.
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    DNA Blunting Tutorial

    The first step in determining how your ends will be blunted is to determine if they are 5´ or 3´ overhangs. This tutorial will teach you how to identify what type of overhang you have, as well as which enzyme will blunt that end, and how.