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Fig. 1. Overview of XY activity during spermatogenesis. Transcriptional
activity of the X and Y chromosomes is shown, with green indicating high
expression and orange indicating low expression. The X and Y chromosomes are
transcriptionally active during type A, intermediate (In) and type B
spermatogonial divisions. They remain active during leptotene and zygotene,
although total nuclear transcription is low during this time. On entry into
pachytene, BRCA1 and ATR coat the entire length of the unsynapsed regions of
the X and Y axial elements, followed by the translocation of ATR to the
surrounding chromatin, where H2AX phosphorylation and MSCI takes place.
Following meiosis, X and Y chromosome repression is maintained and the X and Y
chromosomes appear as heterochromatic domains called post-meiotic sex
chromatin (PMSC). This is shown as the light-turquoise structure located next
to the dark-turquoise chromocentre - the site at which centromeres cluster.
Elong., elongated; sec., secondary.