Structures and functions of band boundaries of mammalian chromosomes

哺乳动物染色体的带区与基因组的各种特征有关，如S期的DNA复制时间和长距离GC%镶嵌结构。因此，条带边界被认为对应于复制时间和GC%水平的过渡区域。在本研究中，为了精确地确定复制转换点，我们确定了XIC及其周围的MHC及其周围非MHC区域的复制时间（X失活中心），以及PAB内和周围在染色体步移和测序后，检测了染色体（假常染色体边界）;然后检查了与GC%水平和染色体带区的相关性。在II类和III类MHC、I类和其端粒非MHC、II类和其着丝粒非MHC之间的连接处发现了复制时间和GC%水平的一致转换。在XIC的XIST基因座的3'部分也发现了复制开关，其被认为对应于Xq13.2和q13.3之间的边界。在这些连接区域内，存在包括具有三链体形成潜力的多嘌呤/多嘧啶序列的特征序列。用抗三链体抗体免疫染色和用聚嘌呤/聚嘧啶探针进行非变性FISH，发现三链体与人间期核中的单个着丝粒在空间上是序列特异性相关的。

Band zones of mammalian chromosome are associated with various genome characteristics such as DNA replication timing during S phase and long-range GC% mosaic structures. Therefore, band boundaries are thought to correspond to transition regions for replication timing and GC% levels, In this study, to identify replication switch point precisely, we determined replication timing in MHC and its surrounding non-MHC regions in and around XIC (X inactivation center), and in and around PAB (pseudoautosomal boundary), after chromosome walking and sequencing ; then correlation with GC% levels and chromosome band zones was examined. Concordant transitions for replication timing and GC% level were found in the junctions between MHC classes II and III, between class I and its telomeric non-MHC, and between class II and its centromeric non-MHC.The junction between class I and its telomeric non-MHC was assigned cytogenetically as the border between 6p21.3 and p22.1. The replication switch was also found in the 3' portion of XIST locus in XIC, that is thought to correspond to the border between Xql3.2 and q13.3, Within these junction regions, there are characteristic sequences including polypurine/polypyrimidine tracts with triplex-forming potential Triplexes are known to occur readily in polypurine/polypyrimidine sequences and to pause replication-fork movement in vitro. Using immunostaining with antitriplex antibodies and non-denatured FISH 'with polypurine/polypyrimidine probes, triplexes were found to be sequence-specificaly associated spatially with individual centromeres in the human interphase nucleus.