Watson and Crick determined that DNA is a polymer of nucleotides arranged in a double helix. Each nucleotide is composed of a phosphate group, a sugar (deoxyribose), and a base. There are a total of four bases: adenine, thymine, cytosine, and guanine. Adenine and guanine are purines while cytosine and thymine are pyrimidines. Base-pairing occurs between adenine and thymine and between guanine and cytosine.
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Additionally, a strand of DNA has a 5' end and a 3' end (determined by the attachment of the phosphate to deoxyribose). In the double helix, each strand of DNA runs antiparallel (in the opposite direction) to the other so that the 5' end on one strand is across from the 3' end of the other.
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