Nucleic Acids: Structure of DNA and RNA

Nucleic acids are the biomacromolecules responsible for storing, transmitting and expressing genetic information in every living cell, and are themselves polymers built from repeating monomer units called nucleotides.

Each nucleotide is composed of three distinct chemical parts: a nitrogenous base, a five-carbon (pentose) sugar, and one or more phosphate groups. The nitrogenous bases fall into two chemical categories -- the purines, adenine and guanine, built around a fused double-ring structure, and the pyrimidines, cytosine, thymine and uracil, built around a single ring. When a nitrogenous base is joined to the pentose sugar alone, without any phosphate group, the resulting unit is called a nucleoside; the addition of one or more phosphate groups to a nucleoside produces a nucleotide, and it is nucleotides, joined one to the next through phosphodiester bonds between the phosphate of one unit and the sugar of the next, that form the long backbone of a nucleic acid chain.

The two classes of nucleic acid are distinguished chiefly by their sugar and by one of their four bases. DNA (deoxyribonucleic acid) contains the sugar deoxyribose, which lacks a hydroxyl group at its 2' carbon, and uses the base thymine in place of uracil; its four bases are adenine, guanine, cytosine and thymine. RNA (ribonucleic acid) contains the sugar ribose, retaining the 2' hydroxyl group, and uses uracil in place of thymine; its four bases are adenine, guanine, cytosine and uracil.

The three-dimensional structure of DNA was resolved by James Watson and Francis Crick in 1953, building on the X-ray diffraction data of Rosalind Franklin and Maurice Wilkins, as a double helix: two polynucleotide strands, running in opposite (antiparallel) directions, wound around a common central axis in a right-handed coil, with roughly ten base pairs per complete turn. The sugar-phosphate backbones of the two strands lie on the outside of the helix, while the nitrogenous bases project inward and pair with one another according to a strict rule of complementary base pairing -- adenine always pairs with thymine through two hydrogen bonds, and guanine always pairs with cytosine through three hydrogen bonds -- so that the sequence of bases on one strand precisely dictates the sequence on the other, providing the structural basis for both the storage and the accurate copying of genetic information. …