5-AZADEOXYCYTIDINE DISTINGUISHES BETWEEN ACTIVE AND INACTIVE X-CHROMOSOME CONDENSATION

Treatment with 5-azadeoxycytidine (5-aza-dC) causes dramatic inhibitio n of mitotic condensation in the inactive X chromosome, without affect ing the active X chromosome and autosomes. The undercondensed chromati n structure is most prominent in the late-replicating regions of the i nactive X chromosome, whereas specific earlier-replicating segments on the inactive X probably associated with large blocks of genes that es cape inactivation remain normally condensed. This segmentation of inac tive X chromosomes has been evolutionarily conserved and is unique to mammals. It suggests temporal or mechanistic separation in condensatio n of inactive X chromatin. Condensation of chromatin structure is a no vel and general feature of inactive X chromosomes that may be involved , directly or indirectly, in the maintenance and stabilization of X in activation.