There are two families of conservative site-specific recombinases: the serine recombinases and the tyrosine recombinases.Fundamental to the mechanism used by both families is that when they cleave the DNA, a covalent protein–DNA intermediate is generated.For the serine recombinases, the side chain of a serine residue within the protein’s active siteattacks a specific phosphodiester bond in the recombination site (Fig. 12-5).