Circularization of P1 DNA. (a) P1 phage contains about 1.4 copies of its genome, packaged in its head. In a host cell, circularization generates monomeric, circular genomes. The orange arrows are loxP sites and serve to define one monomeric genome equivalent. Circularization may involve Cre-mediated recombination or homologous recombination at other locations. (b) Following circularization, the P1 genome can undergo rolling-circle replication, producing many copies from one template.