Any map that shows the arrangement of the material (i.e. the nucleoprotein) making up a chromosome or segment of a genome (see chromosome map) and the physical distance between various genetic sites (loci), ideally measured in nucleotides (nt) or base pairs (bp). There are several types of physical map, differing vastly in scale and detail. Moreover, a wide range of mapping techniques and strategies are available, the choice depending on the extent and detail required and existing knowledge of the organism’s genome. The coarsest physical maps are ones depicting chromosome banding patterns, which are dark and light transverse bands obtained by staining entire chromosomes in mitosis. These cytological maps enable characterization of individual chromosomes and can reveal gross anomalies, such as missing or duplicated segments. The chromosomal location of a particular DNA base sequence of interest can be determined using a DNA probe and fluorescence in situ hybridization (FISH). Restriction mapping reveals the order and distance apart of sites cleaved by restriction enzymes; such sites are important genetic markers, used in pinpointing mutations. On a larger scale are contig maps; these show the order of cloned DNA segments taken from a DNA library and fitted together to form a series of overlapping, or contiguous, segments, called a contig. Such segments are roughly on a gene-length scale. Once a contig has been correctly aligned, the base sequence of each component segment can be determined (see dna sequencing), and hence the overall sequence of the chromosomal DNA can be pieced together. Useful markers in establishing contigs are short unique sequences called sequence-tagged sites. Genes that are expressed in a certain tissue or disease can be identified by searching databases for expressed sequence tags, which are unique markers of complementary DNA. These can be used to construct a probe to identify a particular gene’s location. Other useful markers in constructing physical maps include variable number tandem repeats (minisatellite DNA) and short tandem repeats (microsatellite DNA). The advent of fast, inexpensive DNA sequencing coupled with computerized analytical techniques and access to a fast-growing body of sequence information held on databases is rapidly changing approaches to genetic mapping. Compare linkage map.