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).
An unbalanced Y to X translocation due to a de novo mutation is described in a female with some clinical features of the Turner syndrome. Her karyotype is defined as 46,X,t(X;Y)(Xp11.2;Yq11). Hae III restriction analysis revealed an amount of male-specific DNA sequences in the normal male range. DNA replication analysis showed that in all cells studied the translocation X chromosome was late replicating and that the X segment of the translocation chromosome was later replicating while replication of the Y segment varied. A serologic test indicated a reduced titer of H-Y antigen, and biochemical studies for the enzyme steroid sulfatase revealed activity in the male range.
High resolution patterns of DNA replications in human lymphocyte chromosomes during early and late S-phases were studied by means of the BrdU-Hoechst-Giemsa technique. The late replicating bands were found to be identical with highly detailed G-bands. Between early replicating bands and R-bands subtile differences were observed. A possible correlation between a replication band seen on the chromosomal level and a replication cluster observed after fiber autoradiography is discussed.
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