In chemistry, describing an atom or group that is attached to a ring structure but not part of the ring itself.
From Greek 'exo-' (outside) and 'kyklos' (circle/ring). The term developed in organic chemistry in the late 19th century to classify molecular structures based on whether atoms are inside or outside ring formations.
DNA's famous double helix contains exocyclic atoms in its sugar-phosphate backbone—they stick out from the ring structures of the sugars, like branches on a tree, and this helps scientists understand how DNA actually works.
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