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  • It would also be interesting to understand the potential con


    It would also be interesting to understand the potential connection between asymmetric histone inheritance and another phenomenon reported by several investigators: selective DNA strand segregation (reviewed by Evano and Tajbakhsh, 2013, Rando, 2007, Tajbakhsh and Gonzalez, 2009). Recent development of the chromosome orientation fluorescence in situ hybridization (CO-FISH) technique (Falconer et al., 2010) allows study of selective chromatid segregation at single-chromosome resolution. Using this technique in mouse satellite cells, it has been demonstrated that all chromosomes are segregated in a biased manner, such that pre-existing template DNA strands are preferentially retained in the daughter cell that retains stem cell identity. Interestingly, this biased segregation becomes randomized in progenitor non-stem cells (Rocheteau et al., 2012). Using CO-FISH in Drosophila male GSCs, sex chromosomes have been shown to segregate in a biased manner. Remarkably, sister chromatids from homologous (S)-Mephenytoin have been shown to co-segregate independent of any specific strand preference (Yadlapalli and Yamashita, 2013). Such findings hint at a possible epigenetic source guiding the coordinated inheritance of Drosophila homologous autosomes. In many cases of biased inheritance, researchers have speculated about the existence of a molecular signature that would allow the cell to recognize and segregate sister chromatids bearing differential epigenetic information (Klar, 1994, Klar, 2007, Lansdorp, 2007, Rando, 2007, Yennek and Tajbakhsh, 2013). However, the identity of such a signature has remained elusive. The work represented in this paper provides experimental evidence demonstrating that a tightly-controlled histone modification, H3T3P, is able to distinguish sister chromatids and coordinate their segregation.
    Experimental Procedures
    Author Contributions
    Results and Discussion
    Results and Discussion
    Experimental Procedures Unless otherwise indicated, a prometaphase arrest was induced by culturing U2OS cells consecutively for 24 hr with 2 mM thymidine, 2 hr without thymidine, and 16 hr with 100 ng/ml nocodazole. The arrested cells were harvested by shake off. A mitotic release was induced by culturing the arrested cells for 60–120 min in medium without nocodazole. To produce chromosome spreads, we treated mitotically arrested cells for 15 min at 37°C with 75 mM KCl before fixation. Images with a single optical section were acquired at 21°C with a Zeiss 510 META laser-scanning confocal microscope equipped with a Plan Neofluar 40 × 1.3 NA oil differential interference contrast (DIC) objective (for Figure 3) or Plan Apochromat 63 × 1.40 NA oil DIC objective (for Figure 4 and Figure 5). Images were deconvoluted with Zeiss 510 image software. Final images were processed and assembled using Photoshop CS3 (Adobe). Brightness and contrast were adjusted using only linear operations applied to the entire image. For quantification, Z stack scans were performed through each cell for 13–16 sections with 1 μm intervals. Signals of H3T3ph and DNA were processed with ImageJ 1.43u software (National Institutes of Health) using the “sum slices” feature of z project. After subtraction of the background signal, the H3T3ph/DNA ratio was calculated and plotted with Origin 8.1 software (OriginLab software).
    Meiosis: A Specialized Cellular Division Meiosis differs fundamentally from mitosis because, through two subsequent divisions, and no intervening round of DNA replication, haploid gametes form that are genetically and biochemically distinct from the starting diploid precursor cell. Homologous recombination (see Glossary) and reduction of the chromosome content in half generates genetically distinct cells. Coupled to meiosis is the formation of gametes (gametogenesis), a developmental program that varies between species and sex that makes the gamete biochemically distinct from the starting cell. The first meiotic division (meiosis I, MI) is unique because homologous chromosomes segregate while sister chromatids remain associated with one another. The subsequent meiotic division (meiosis II, MII) resembles that of mitosis where sister chromatids segregate.