A Code for All Life

The principle of hereditary transmission is a central tenet of life on earth: all organisms inherit a structural and functional organization from their progenitors. What is inherited by an offspring is not an exact copy of the parent but a set of coded instructions that a developing organism uses to construct a body resembling its parents. These instructions are in the form of genes, the fundamental units of inheritance. One of the great triumphs of modern biology was the discovery in 1953 by James Watson and Francis Crick of the nature of the coded instructions in genes. The genetic material (deoxyribonucleic acid, DNA) is composed of nitrogenous bases arranged on a chemical chain of sugar-phosphate units. The genetic code lies in the linear order or sequence of bases in the DNA strand.

Because the DNA molecules replicate and pass from generation to generation, genetic variations can persist and spread in a population. Such molecular alterations, called mutations, are the ultimate source of biological variation and the raw material of evolution.