A-D. Spread spermatocytes at different stages of prophase-I labelled with antibodies against SYCP3 (green), centromeres (magenta) and RNA polymerase-II (blue). Non-homologous segments of the sex chromosomes (X, Y) and the neo-PAR are indicated. A. RNA pol-II is absent from the nucleus during zygotene. B. At mid pachytene, a weak RNA pol-II signal is detected in several areas of the nucleus. C. By late pachytene, the nuclear RNA pol-II signal has greatly increased and is maintained through diplotene (D). In contrast to the neo-PAR and autosomes, the non-homologous segments of the sex chromosomes (X, Y) are mostly or completely devoid of RNA pol-II. E-H. Spread spermatocytes at different stages of prophase-I labelled with antibodies against SYCP3 (green) and ATR (magenta). E. At zygotene, ATR foci are observed along the unsynapsed regions of the autosomes and sex chromosomes. F. At mid pachytene, ATR is only seen on the unsynapsed regions of the sex chromosomes. At late pachytene (G) and diplotene (H), the ATR signal has extended to the chromatin of the non-homologous segments of the sex chromosomes. The neo-PAR is not labelled by ATR.