title: Cytologic evidence for three human X-chromosomal segments escaping inactivation.
abstract: Early replication of prometaphasic human sex chromosomes was studied with the bromodeoxyuridine (BrdU)-replication technique. The studies reveal that two distal segments of Xp, including bands Xp 22.13 and Xp 22.3, replicate early in S-phase and therefore may not be subject to random inactivation. Furthermore, the replication of these distal segments of Xp occurs synchronously with those of the short arm of the Y chromosome including bands Yp 11.2 and Yp 11.32. These segments of Xp and Yp correspond well to the pairing segment of the X and Y chromosomes where a synaptonemal complex forms at early pachytene of human spermatogenesis. The homologous early replication of Yp and the distal portion of Xp may be interpreted as a remnant left untouched by the differentiation of heteromorphic sex chromosomes from originally homomorphic autosomes. A third early replicating segment is situated on the long arm of the X chromosome and corresponds to band Xq 13.1. This segment may be correlated with the X-inactivation center postulated by Therman et al. (1979).
mesh_terms: D001973:Bromodeoxyuridine / Q000494:pharmacology; D004261:DNA Replication / Q000187:drug effects; D005260:Female; D006801:Humans; D007621:Karyotyping; D007713:Klinefelter Syndrome / Q000235:genetics* / Q000473:pathology; D008214:Lymphocytes / Q000187:drug effects / Q000502:physiology; D008297:Male; D012730:Sex Chromosomes / Q000648:ultrastructure*; D014960:X Chromosome / Q000648:ultrastructure*; D014998:Y Chromosome / Q000648:ultrastructure*
chemical_list: D001973:Bromodeoxyuridine