Decondensation of pericentric heterochromatin alters the sequence of centromere separation in mouse cells

The centromeres of a genome separate in a sequential, nonrandom manner that is apparently dependent upon the quantity and quality of pericentric heterochromatin. It is becoming increasingly clear that the biological properties of a centromere depend upon its physicochemical makeup, such as its tertiary structure, and not necessarily on its particular nucleotide sequence. To test this idea we altered the physical state of the AT-rich pericentric heterochromatin of mouse with Hoechst 33258 (bis-benzimidazole) and studied a biological parameter, viz., sequence of separation. We report that an alteration in the physical state of heterochromatin, i.e., decondensation, is accompanied by aberrations in the pattern of centromere separation. The most dramatic effect seems to be on chromosomes with large blocks of heterochromatin. Many chromosomes with large blocks of heterochromatin that, in untreated cells, separate late tend to separate early. Decondensation with Hoechst 33258 does not seem to alter the sequence of separation of inactive centromeres relative to that of active centromeres. These data indicate that alteration in the physical parameters of the pericentric heterochromatin might dispose the centromeres to errors. It is likely that this aberration results from early replication of the pericentric heterochromatin associated with active centromeres.