In situ DNA sequence mapping with surface-spread mouse pachytene chromosomes.

Surface-spread pachytene chromosomes are several times the length of metaphase chromosomes and the decondensed chromatin loops are attached to a well-defined axis (Weith and Traut, 1980). This arrangement permits detailed DNA sequence localization by in situ hybridization. We show that two probes to low-frequency repeated sequences (20 to 50 copies) which locate the centromere proximal in the mouse X metaphase chromosome between bands A1 and A3 (Disteche et al., 1985) and which map 5.5 cM apart (Disteche et al., 1989), hybridize to two distinct chromatin regions 3 to 5 microns apart on a 25 microns long pachytene X chromosome core.