Released chromatin: linearized DNA for high resolution fluorescence in situ hybridization.

Free DNA was prepared from routinely harvested and fixed cells for high resolution FISH mapping using either a sodium hydroxide/ethanol mixture or 70% formamide. Hybridization signals from cosmid probes appeared as extended lines. The average length of signals on DNA prepared with sodium hydroxide was significantly greater than with formamide. A set of overlapping cosmids from the HLA class II region was used to determine how precisely the actual overlap or gap between probes can be calculated from the measured overlap or gap between their signals. Lengths of the probe signals and their known kilobase lengths were used as an internal ruler. The mean values calculated from the measured length from 30 or more signals for each probe pair showed remarkable conformity with the known kilobase lengths of their overlaps and gaps. Immediately adjacent probes could also be ordered on the released DNA. These simple procedures dramatically increase the speed with which relationships between probes can be determined during contig construction.