Antiparallel Strand Orientation
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State
DNA's two strands run antiparallel, one oriented 5' to 3' and its partner oriented 3' to 5' across the same helix.
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A phosphate on one strand's 5' carbon end faces its partner strand's 3' hydroxyl end at each end of the helix.
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Antiparallel orientation is not the same fact as complementary base pairing — a helix needs both to hold its shape.