No strand left behind

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Science  28 Sep 2018:
Vol. 361, Issue 6409, pp. 1311-1312
DOI: 10.1126/science.aav0871

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Semiconservative replication of DNA faithfully transmits genetic information, but the copying of epigenetic information, which distinguishes cell identity and is embedded in chromatin, is more complicated: Unwinding and separating the parental double-stranded DNA for replication displaces all chromatin proteins, including the histones that package eukaryotic DNA into nucleosomes. This necessitates that chromatin organization be reestablished after every round of DNA replication. Furthermore, bound chromatin proteins must double after DNA replication, as one parental DNA chromatid becomes two daughters. The rules governing the distribution of histones to two daughter chromatids have long been the subject of speculation. However, on pages 1389 and 1386 of this issue, Petryk et al. (1) and Yu et al. (2), respectively, show that histone distribution is inherently asymmetric, but they identify mechanisms of chromatin regulation to achieve nearly equal distribution of parental nucleosomes to daughter chromatids. These findings have implications for how epigenetic information is propagated through cell divisions.