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Histones package and compact DNA by assembling into nucleosome core particles. Most histones are synthesized at S phase for rapid deposition behind replication forks. In addition, the replacement of histones deposited during S phase by variants that can be deposited independently of replication provide the most fundamental level of chromatin differentiation. Alternative mechanisms for depositing different variants can potentially establish and maintain epigenetic states. Variants have also evolved crucial roles in chromosome segregation, transcriptional regulation, DNA repair, and other processes. Investigations into the evolution, structure, and metabolism of histone variants provide a foundation for understanding the participation of chromatin in important cellular processes and in epigenetic memory. Outline 1 DNA is packaged by architectural proteins in all organisms 2 Eukaryotic core histones evolved from archaeal histones 3 Bulk histones are deposited after DNA replication 4 Variant histones are deposited throughout the cell cycle 5 Centromeres are identified by a special H3 variant 6 The replacement histone variant H3.3 is found at active chromatin 7 H3.3 functions in the germline 8 Phosphorylation of H2A.X functions in DNA double-strand break repair 9 H2A.Z plays diverse roles in chromatin regulation 10 H3.3 and H2A.Z occupy discrete chromatin locations 11 H2A.Z nucleosome occupancy is dynamic and changes the properties of chromatin 12 H2A.Z functions in epigenetic inheritance 13 Other H2A variants differentiate chromatin, but their functions are as yet unknown 14 Many histones have evolved to more tightly package DNA 15 Histone variants and human disease 16 Conclusions and future research
Henikoff et al. (Thu,) studied this question.