Lambda is a temperate Escherichia coli bacteriophage. The virion DNA is linear and double-stranded (48502 bp) with 12 bp single-stranded complementary 5’-ends. After the phage particle injects its chromosome into the cell, the chromosome circularizes by end joining. In the lytic pathway, phage genes encoding replication, lysis, and virion proteins are expressed. The chromosome replicates, and the replicas are cleaved and packaged into progeny phage particles. In the lysogenic pathway, phage gene expression is repressed, and the circular chromosome inserts into the bacterial chromosome by recombination.