Double helix
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- For other meanings of double helix, see Double helix (disambiguation)
In geometry a double helix (plural helices) typically consists of two congruent helices with the same axis, differing by a translation along the axis, which may or may not be half-way.
In modern pop culture, the double helix shape is strongly associated with DNA. That the double helix is the structure of DNA was first published by James D. Watson and Francis Crick in 1953. They constructed a molecular model of DNA in which there were two complementary, antiparallel (side-by-side in opposite directions) strands of the bases guanine, adenine, thymine, and cytosine, covalently linked through phosphodiester bonds. The four nitrogen-containing bases found in DNA are divided into two groups: purines and pyrimidines. Two-ringed bases are purines. One-ringed bases are called pyrimidines. Adenine and guanine are purines, whilst thymine and cytosine are pyrimidines. Each strand forms a helix, and the two helices are held together through hydrogen bonds, ionic forces, hydrophobic interactions, and van der Waals forces forming a double helix.
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