[
Methods Cell Biol,
1995]
Both the localization and distribution of nucleic acid sequences in genomes and in cells can be visualized by hybridization of labeled probe DNAs to cytological preparations of chromosomes or tissues. With the introduction of nonisotopically labeled nucleotides that could be incorporated into cloned DNAs by enzymatic methods in vitro, it became possible to detect the site of hybridization quickly using antibodies that recognized the modifying group on the nucleotides incorporated into the probe DNA. More recently, nucleotides labeled with a fluorescent molecule have been incorporated into probes by invitro enzymatic reactions and the site of hybridization can then be visualized directly. As fluorescence in situ hybridization provides a rapid and high-resolution method for mapping genes, it is being sued increasingly for mapping cloned DNAs to chromosomes and for the ordering of clones in large-scale genome projects. On the other hand, physically mapped clones can also be used to label chromosomes for analysis of such biological processes as chromosome segregation, pairing in meiosis, and interphase nuclear order. Nonisotopic methods of hybridization are also ideally suited to visualization of mRNA distributions in tissues, because the signal can be detected in thick specimens, in contrast to isotopic methods that require thin specimens for detection by autoradiography...