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 to 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.