The cells adherence to these base-pairing rules ensures that the new strand is an exact copy of the old one. Each DNA molecule contains many genes the basic physical and functional units of heredity. A gene is a specific sequence of nucleotide bases, whose sequences carry the information required for constructing proteins, which provide the structural components of cells and tissues as well as enzymes for essential biochemical reactions. The human genome is estimated to comprise at least 100,000 genes. Interspersed within many genes are intron sequences, which have no coding function.
Proteins are large, complex molecules made up of long chains of subunits called amino acids.
Twenty different kinds of amino acids are usually found in proteins. For example, the base sequence ATG codes for the amino acid methionine. The genetic code is thus a series of codons that specify which amino acids are required to make up specific proteins. This mRNA is moved from the nucleus to the cellular cytoplasm, where it serves as the template for protein synthesis. The utility of this strategy is described in the section on physical mapping.