physical map

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physical map (in genetics) 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). There are several types of physical map, differing vastly in scale and detail. 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. On a much 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. Compare linkage map.