Agarose electrophoresis gel used in mapping" DNA extracted from chromosomes of the bacteria Escherischia coli. The light pink colour is due to ethidium bromide, a stain that fluoresces in ultra- violet light. DNA mapping refers to a physical survey of each of an organism's chromosomes in an attempt to locate genes or other landmarks. Mapping and sequencing (decoding the DNA base-pair sequences of chromosomes) are the two phases of the human genome project, an ambitious plan to reveal all of the information encoded in the 23 pairs of human chromosomes. Photographed in Dr Jonathan Beckwith's laboratory at Harvard, USA."