DNA or deoxyribonucleic acid is the name for the molecule that contains the genetic information in all living things. This molecule consists of two strands that wind around each other to form a double helix structure.

The basic unit of nucleic acids are called nucleotides, which are organic molecules formed by the covalent bonding of a nucleoside (a pentose which is a type of sugar and a nitrogenous base) and a phosphate group. So each nucleotide is made up of a pentose sugar called deoxyribose, a nitrogenous base which can be adenine (A), thymine (T), cytosine (C) or guanine (G) and a phosphate group.

<u>What distinguishes one polynucleotide from another is the nitrogenous base</u>, and thus the sequence of DNA is specified by naming only the sequence of its bases. The sequential arrangement of these four bases along the chain is what encodes the genetic information, following the following criterion of complementarity: A-T and G-C. So the sequence of these bases along the chain is what encodes the instructions for forming proteins and RNA molecules. In living organisms, DNA occurs as a double strand of nucleotides, in which the two strands are linked together by connections called hydrogen bridges.

The chemical convention of naming the carbon atoms in the pentose nucleotide pentose numerically confers the names 5' end and 3' end ("five prime end" and "three prime end" respectively). The 5'-end designates the end of a DNA strand that coincides with the phosphate group of the fifth carbon of the respective terminal deoxyribose. A phosphate group attached to the 5'-end allows the ligation of two nucleotides; for example, the covalent bonding of the 5'-phosphate group to the 3'-hydroxyl group of another nucleotide, to form a phosphodiester bond.

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