  Drag reduction by polymers in turbulent wall-bounded flows exhibits universal
and non-universal aspects. The universal maximal mean velocity profile was
explained in a recent theory. The saturation of this profile and the crossover
back to the Newtonian plug are non-universal, depending on Reynolds number Re,
concentration of polymer $c_p$ and the degree of polymerization $N_p$. We
explain the mechanism of saturation stemming from the finiteness of
extensibility of the polymers, predict its dependence on $c_p$ and $N$ in the
limit of small $c_p$ and large Re, and present the excellent comparison of our
predictions to experiments on drag reduction by DNA.
