6533b86efe1ef96bd12cc66e

RESEARCH PRODUCT

Mapping of Polytene Chromosomes

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description

Principle and Polytene chromosomes consist of up to several thousands of chromatids applications and are therefore especially suitable for direct mapping with the help ofIn situhybridization. With the introduction of nonradioactive labeling and detection methods, e.g., fluorescenceIn situhybridization (FISH) (Lan-ger-Safer et al., 1982), theIn situhybridization procedure has become easy to perform and the results can be obtained within a day. Furthermore, the method described here (Schmidt et al., 1988; Schmidt, 1992) is a simplified version which additionally allows for the hybridization of more than one DNA probe simultaneously. This double or multicolor hybridization results in very precise mapping of two neighboring DNA probes, provided that these probes are differentially labeled and therefore can be detected by an-ger colors. In this way, it was possible to simultaneously localize two DNA sequences which were only approximately 35 kb apart from each other inChironomus, and less than 20 kb inDrosophila melanogaster(Figure1). Thus, it is easy to determine the orientation of a cloned region of DNA (e.g., a genomic walk) (Kraemer and Schmidt, 1993) and the direction of an-ger of a gene within this region with respect to the centromer and telomer of the chromosome concerned. In addition, multicolor FISH does not only allow for the localization of a DNA probe to a certain an-ger region (or band), but even more precisely with respect to other neighboring DNA probes, which may reside in the same chromosomal band (Kraemer et al., 1998).