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The deoxyribonucleic acid, DNA, is a long chain of nucleotides which consist of Deoxyribose (a pentose = sugar with 5 carbons), Phosphoric Acid and Organic (nitrogenous) bases (Purines - Adenine and Guanine, or Pyrimidines -Cytosine and Thymine). in a nucleotide, the atoms of the organic base are numbered 1, 2, ... and the atoms of the sugar, wether it is a deoxyribose like in DNA or a ribose like in RNA, are numbered 1', 2', 5'. Atoms in the sugar component of a nucleotide provide the link between the base and the phosphate group. The 1' carbon is attached to the 9 nitrogen of a purine, or the 1 nitrogen of a pyrimidine. The OH (hydroxyl) group on the 5' carbon is replaced by a bond to the phosphate group (ester bond). DNA consists of two associated polynucleotide strands that wind together in a helical fashion. It is often described as a double helix. Each polynucleotide is a linear polymer in which the monomers (deoxynucleotides), are linked together by means of phosphodiester bridges , or bonds. These bonds link the 3' carbon in the ribose of one deoxynucleotide to the 5' carbon in the ribose of the adjacent deoxynucleotide. The sugar / phosphate backbone is on the outside while the organic bases project into the inside of the double helix. The organic bases stack on top of each other in parallel plans. By convention a polynucleotide is read from the 5' end to the 3' end. The orientations of the two strands are antiparallel : their 5' - 3' directions are opposite. The base composition is variable, but in all cases the amount of adenine is equal to the amount of thymine (A=T). In the same manner, C=G. Consequently A+C= T+G. This structure was first described by James Watson and Francis Crick in 1953.

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