Practice question

Site-specific recombinases are divided into serine and tyrosine families based on nucleophilic residue forming covalent protein-DNA intermediate during strand exchange. Cre recombinase belongs to tyrosine family alongside lambda integrase, Flp, and XerC/D. Catalytic tyrosine at C-terminus attacks phosphodiester bond generating 3'-phosphotyrosine linkage and free 5'-hydroxyl, proceeding through sequential single-strand exchanges forming Holliday junction intermediate. Tetramer synchronizes two rounds of cleavage and ligation without ATP or external energy, using binding energy and base pairing. Conserved Arg-His-Arg triad assists active site chemistry and transition state stabilization during recombination.