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Inadequate dNTPs

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An incorrect concentration of deoxynucleotidetriphosphates (dNTPs) can cause problems for the PCR procedure. The usual dNTP concentration is between 40μM and 200μM of EACH of the four dNTPs. Excessive dNTP concentrations can inhibit the PCR preventing the formation of product. However, concentrations up to 400 μM each dNTP have been reported to work adequately. Low primer, target, Taq, and dNTP concentrations are preferable as these generally ensure cleaner product and lower background. For longer PCR-fragments a higher deoxynucleotidetriphosphate concentration may be required. A large change in the dNTP concentration may require a corresponding change in the concentration of MgCl2.

Suboptimal concentration of nucleotides can cause incomplete primer elongation or premature termination of DNA synthesis during the elongation step of the PCR cycle.

from PCR Troubleshooting: The Essential Guide see also PCR Troubleshooting and Optimization: The Essential Guide

Further reading