The Science of Existence (119) Chemical Composition

Chemical Composition

All of today’s DNA, strung through all the cells of the Earth, is simply an extension and elaboration of the first molecule. ~ American physician Lewis Thomas

The polynucleotides of DNA and RNA form a long chain built from millions of nucleotide subunits. Nucleotides form the basic structural unit of both DNA and RNA. A nucleotide has a nucleobase and sugar and phosphate groups, all glued together by ester bonds. A nucleobase is a ring-shaped molecule with a nitrogen base.

Nucleobases pair up by chemical affinity. Base pairs come in a strictly limited variety of combinations, circumscribed by chemical bonding rules: A–T and C–G for DNA, and A–U and C–G for RNA.

The larger nucleobases – adenine (A) and guanine (G) – belong to the purine chemical class. The smaller – cytosine (C) and thymine (T) and uracil (U) – are in the pyrimidine class. Whereas pyrimidines are simple ring molecules, purines have fused rings. Purines and pyrimidines are complementary by their sharing hydrogen bonds, which is particularly convenient for adaptable information storage because hydrogen bonds are easily broken.

The backbone of both DNA and RNA is provided by a phosphodiester bond: a group of strong covalent bonds between 2 5-carbon-ring sugars and a phosphate group, over 2 ester bonds.

Esters are ubiquitous organic compounds, formed by condensing an acid with an alcohol. Many lipids are fatty-acid esters of glycerol. Esters with low molecular weight are found in pheromones, the chemical compound used in scent-based communication.