  In the framework of the Poland Scheraga model of DNA denaturation, we derive
a recursion relation for the partition function of double stranded DNA,
allowing for mismatches between the two strands. This relation is studied
numerically using standard parameters for the stacking energies and loop
entropies. For complementary strands (of length 1000), we find that mismatches
are significant only when the cooperativity parameter $\sigma$ is of order one.
Since $\sigma \sim O(10^{-5})$ in DNA, entropic gains from mismatches are
overwhelmed by the energetic cost of opening a loop: mismatches are therefore
irrelevant (and molecular recognition of the strands is perfect). Generating
random mutations with probability $p$ on one strand, we find that large values
of $\sigma$ are rather tolerant to mutations. For realistic (small) values of
$\sigma$, the two strands do not recombine, even for small mutation fractions.
Thus, molecular recognition is extremely selective.
