Replication timing of the human X-inactivation center (XIC) region: correlation with chromosome bands.

The human genome is composed of long-range G+C% mosaic structures, which are thought to be related to chromosome bands. Replication timing during S phase is associated with chromosomal band zones; thus, band boundaries are thought to correspond to regions where replication timing switches. The proximal limit of the human X-inactivation center (XIC) has been localized cytologically to the junction zone between Xq13.1 and Xq13.2. Using PCR-based quantification of the newly replicated DNA from cell-cycle fractionated THP-1 cells, the replication timing in and around the XIC was determined at the genome sequence level. We found two regions where replication timing changes from the early to late period during S phase. One is located near a large inverted duplication proximal to the XIC, and the other is near the XIST locus. We propose that the 1Mb late-replicated zone (from the large inverted duplication to XIST) corresponds to a G-band Xq13.2. Several common characteristics were observed in the XIST region and the MHC class II-III junction which was previously defined as a band boundary. These characteristics included differential high-density clustering of Alu and LINE repeats, and the presence of polypurine/polypyrimidine tracts, MER41A, MER57 and MER58B.