Human genome research: scientist reading the base- pair sequence from a DNA sequencing autoradiogram of human chromosomes. The letters A,T,C & G represent adenine, thymine, cytosine & guanine - the sequence of which varies along the length of the DNA molecule. DNA sequencing is most usefully performed on sections of DNA called genes that encode specific proteins. Sequencing and mapping - surveying each of the 23 pairs of human chromosomes to locate genes or other important markers - are two phases in the human genome project. The making of a complete genetic map involves a biochemical survey of every gene expressed on all 23 pairs of human chromosomes.