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The cos region of a lambda concatemer. Upper panel: The cosQ, cosN and cosB subsites within a cos site in concatemeric DNA. The cosB subsite is composed of the I1 and R-elements, as indicated. The I2 region lies between cosN and the R3 element. Middle panel: The nucleotide sequence of cosN, with the cosNL and cosNR half-sites indicated. The center of symmetry of cosN is indicated with a dot. Terminase normally nicks the duplex at N₁ and N₂ sites indicated with arrows. In the absence of ATP, terminase incorrectly nicks the duplex at N_x and/or N_y sites. Lower panel: Strand separation by terminase yields the matured D_R and D_L ends of the lambda genome, as shown.