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Why is it necessary to add dNTPs when using the 3´ exonuclease activity of T4 DNA Polymerase to convert a 3´ overhang to a blunt end?

T4 DNA Polymerase (Cat.# M4211, M4215) is often the enzyme of choice for removing single-stranded 3´ overhangs from double-stranded DNA fragments due the enzyme's relatively strong 3´- to 5´-exonuclease activity. In the absence of dNTPs (Cat.# U1240, U1330), the strong exonuclease activity can degrade double-stranded DNA in addition to the single-stranded overhang. Adding 100µM dNTPs will cause the T4 DNA Polymerase to stop at the double-stranded DNA and only digest the overhang to produce a blunt end. The DNA polymerase I large Klenow fragment also has a 3´- to 5´-exonuclease activity and can be used to remove a 3´ overhang. However, the 3´- to 5´-exonuclease activity is much weaker, and thus Klenow is not commonly used for this application.

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