Semiconservative DNA replication ensures the faithful duplication of genetic information during cell divisions. However, how epigenetic information carried by histone modifications propagates through mitotic divisions remains elusive. To address this question, the DNA replication-dependent nucleosome partition pattern must be clarified. Here, we report significant amounts of H3.3-H4 tetramers split in vivo, whereas most H3.1-H4 tetramers remained intact. Inhibiting DNA replication-dependent deposition greatly reduced the level of splitting events, which suggests that (i) the replication-independent H3.3 deposition pathway proceeds largely by cooperatively incorporating two new H3.3-H4 dimers and (ii) the majority of splitting events occurred during replication-dependent deposition. Our results support the idea that "silent" histone modifications within large heterochromatic regions are maintained by copying modifications from neighboring preexisting histones without the need for H3-H4 splitting events.

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