Intracellular ratio of rNTP/ dNTP is very high.
DNA polymerase: It actively discriminates against rNTPs via a “STERIC gate” formed by the Glu and Phe side chains, which excludes 2′-OH group of rNTP.
But because of high concentration of rNTPs, they still get incorporated into DNA by Pol δ and ε. rNTP incorporation rate is 1 in 1000 dNTP. But they are recognized by mismatch and continuously removed by respective polymerases via intrinsic 3′-exonuclease activity (proofreading).
RNA polymerase: While DNA pol negatively selects rNTPs and removes them, RNA pol positively selects rNTPs. It recognizes ribose via formation of “HYDROGEN BOND” between -OH group of Tyr 639 of RNA Pol and 2′-OH group of rNTP.