Mapping of DNAase I sensitive regions on mitotic chromosomes

We have shown that in fixed mitotic chromosomes from female G. gerbillus cells the inactive X chromosome is distinctly less sensitive to DNAase I than the active X chromosome, as demonstrated by in situ nick translation. These results indicated that the specific chromatin conformation that renders potentially active genes sensitive to DNAase I is maintained in fixed mitotic chromosomes. We increased the sensitivity and accuracy of in situ nick translation using biotinylated dUTP and a specific detection and staining procedure instead of radioactive label and autoradiography and now show that in both human and CHO chromosomes, the DNAase I sensitive and insensitive chromosomal regions form a specific dark and light banding pattern. The DNAase I sensitive dark D-bands usually correspond to the light G-bands, but not all light G-bands are DNAase I sensitive. Identifiable regions of inactive constitutive heterochromatin are in a DNAase I insensitive conformation. Our methodology provides a new and important tool for studying the structural and functional organization of chromosomes.