DNA double helix.
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   A  DNA  molecule  is usually double-stranded, with the sugar-phosphate
   backbone of two polynucleotide strands on the outside of the helix.
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   Holding  the  two  strands  together  are  pairs  of nitrogenous bases
   attached to each other by hydrogen bonds.
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   Adenine (A) can pair only with Thymine (T).
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   Guanine (G) can pair only with Cytosine (C).