A concatemer is a polymeric DNA sequence which contains many repeats of the same DNA sequence. They are seperated by cos sites (protein binding nucleotide sequence) which only occur once in each copy of the genome. For example if the genes were arranged CAT, the concatemer would be CATCATCATCAT...etc.
Concatemers can be beneficial within DNA cloning when using gamma-phage. The concatemers contain no gamma DNA but are able to be packaged in vitro as long as a cos site is present and the correct cos spacing is available. The phage cuts the concatemer and packages it.
The complementary single - stranded ends anneal in a reaction which is favoured thermodynamically. After annealing, it is joined covalently and catalyzed by the host DNA Ligase which then proceeds to form phosphodiester bonds.