Practice question

Cellular rNTP concentrations exceed dNTPs roughly tenfold, yet DNA polymerases insert ribonucleotides rarely, about once per thousand nucleotides. Selectivity depends on steric gate residue in active site. In E. coli Pol I this is Glu-710 backed by Phe-762 creating clash with 2' hydroxyl of ribose sugar. Family B and Y polymerases use tyrosine or phenylalanine similarly blocking ribose. The clash prevents proper alignment of alpha-phosphate, raising energetic barrier for catalysis. Mutating gate dramatically increases rNMP incorporation causing alkali-sensitive strand breaks threatening genome stability.