Figure 8. Timing and requirements for CID assembly in mitosis and meiosis.

While newly synthesized canonical histones are assembled during DNA replication in S phase, in most organisms the centromeric histone CENP-A is assembled outside of S phase, in mitosis or G1 phase. Drosophila CID is assembled at metaphase in cultured S2 cells [13], at anaphase in the syncytial divisions in embryos [14], and at telophase/G1 phase in larval brain nonstem and neuroblast stem cells (this study). In neuroblasts, CID loading in the mother stem cell precedes loading in the daughter cell (this study). In human mitotic HeLa cells in culture, CENP-A is assembled at late telophase/early G1 phase [10]. In meiosis in flies, CID is assembled at centromeres during prophase of meiosis I and after exit from meiosis II in spermatids (this study). The putative HJURP functional homolog in flies, CAL1, is required for CID loading in S2 cultured cells, in embryos, and in prophase of meiosis I, while HJURP is required for CENP-A loading in human cells [16],[17]. CENP-C is required for CID assembly in S2 cultured cells, embryos [24],[26], and in meiosis (this study). The timing of CENP-A assembly in somatic (stem or nonstem) cells in tissues, in embryonic or meiotic divisions in mammals is currently unknown.