(a) Affinity in Sticky Ended Cohesion. Two double helical strands with complementary overhangs are shown. Under appropriate conditions, they will cohere in a sequence-specific fashion, and can be ligated, if desired. (b) Structure in Sticky-Ends. A portion of the crystal structure of an infinite DNA double helix formed by sticky-ended cohesion is shown. The part cohering by sticky ends is in the red box, whereas the blue boxes surround continuous DNA segments. The DNA in all three sections is B-DNA. (c) A Stable Branched Junction. There is no dyad symmetry flanking the branch point; tetramers, such as the boxed sequences CGCA and GCAA are unique, and there is no TCAG to complement the CTGA flanking the corner.