Molecular Structure of Nucleic Acid
The two ribbons symbolize the two phosphate-sugar chains, and the horizontal rods the pairs of bases holding the chains together firmly. The vertical line marks on the fiber axis. Those two strands of the helix run in the opposite directions, meaning that the 5′ end of one strand is paired up with the 3′ end of its matching strand.
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X-ray crystallography -
Triple helix