Concatemers
 
A concatemer is a multimeric molecule of DNA formed by identical monomers arranged linearly in the same head-to-tail orientation. Closure of a bacteriophage DNA molecule upon entry into the bacterial host cell is a relatively simple process. The complementary single-stranded ends anneal in a reaction that is thermodynamically favored and by conditions that restrict diffusion and keep the ends close to each other. Annealing is followed by covalent joining, catalyzed by the host DNA Ligase, which forms phosphodiester bonds. Opening of the circular DNA molecule, coupled to its encapsidation to form mature phage particles, however, is rather complex.
 Capsid precursors interact with the newly replicated DNA molecules in the form of concatemers (1, 2), which are the product of DNA replication proceeding in two stages. First, the DNA injected into the host is circularized and covalently closed, as previously described. In the case of the lambda phage, the DNA undergoes several stages of DNA replication in which the circular DNA rings generate daughter rings. Then, during phage vegetative growth, rolling-circle replication gives rise to linear concatemers of l DNA. These concatemers, which in the absence of packaging may be up to 10 chromosomal units long, are the normal substrates for packaging.
RNA primers for DNA synthesis are excised during the process. This raises the question of how the extreme ends of linear DNA can be completed. Not only must a small RNA fragment initiate at the 3′ terminal nucleotide of the template strand, but the sequence must be filled in after ribonucleotide removal. This dilemma was resolved using phage T7 DNA, which has the property of having redundant ends. The sequence of about 160 bp found at the left end is repeated exactly at the right end of the molecule. When the linear T7 molecules replicate, they do not generate unit-length progeny molecules, but very long concatemers (units linked end to end) containing the complete genome sequence repeated over and over.
References
1. D. Kaiser and T. Masuda (1973) Proc. Natl. Acad. Sci. USA 70, 260–264. 
2. D. Kaiser, M. Syvanen, and T. Masuda (1974) J. Supramol. Struct. 2, 318–328.