Composition and Structure

The molecular structures of DNA and RNA are very similar. Nucleic acids have a long "backbone" consisting of alternate sugar and phosphate molecules. In RNA the sugar is ribose and in DNA it is deoxyribose. Attached to each sugar molecule is a nitrogen-containing compound that is a base, a compound capable of accepting protons.

In both DNA and RNA there are four major bases. Those in DNA are called adenine, guanine, cytosine, and thymine. RNA has the first three of these plus uracil. The substitution of uracil for thymine as a base material constitutes the chief chemical difference between RNA and DNA.

Each subunit of a nucleic-acid molecule, consisting of a phosphate, sugar, and base, is known as a mononucleotide. A complete nucleic-acid molecule consists of a large number of these mononucleotides (as many as 400,000) linked together to form a polynucleotide.

The size of a molecule is indicated by its molecular weight, which is the sum of the atomic weights of all the atoms in the molecule. Nucleic acids are among the largest known molecules on earth, with molecular weights in the millions. By comparison, the molecular weight of water is 18.

Individual molecules of nucleic acids cannot be seen with optical microscopes, but they can be studied with the electron microscope and other nonoptical magnifying instruments.

In 1951-53 the British molecular biologist F. H. C. Crick and the United States biochemist James D. Watson, in association with the British physicist Maurice H. F. Wilkins, analyzed photographic images produced by X rays directed through DNA molecules. They determined that the DNA in cells is usually in the form of a double stranded helix (the shape of two stretched, intertwined coiled springs), one DNA molecule being wrapped around another. RNA, on the other hand, is usually a single strand.

The backbone, according to the Watson Crick model, forms the coil of the helix, and the bases jut inward toward the axis of the helix. The bases on the two backbones are joined to one another in pairs by relatively weak chemical bonds called hydrogen bonds. The hydrogen bonds linking the bases in DNA are easily broken, so that the two strands can be partially or completely separated. The adenine on one backbone is always paired with thymine on the other, and guanine is always paired with cytosine.