Human genome research: composite photo of an infant overlaid on a computer-enhanced DNA sequencing autoradiogram. DNA sequencing of chromosomes involves decoding the base-pair sequence of sections of DNA - most usefully, those sections called genes which 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 construction of such a complete genetic map involves a detailed biochemical survey of every gene expressed on all 23 pairs of human chromosomes.