Fig. 1

Relationship of replication-dependent and -independent nucleosome assembly to chromatin status during aging. In younger cells, histone chaperones deposit newly synthesized histones in replication-dependent nucleosome assembly and maintain chromatin with ordered nucleosome positioning throughout multiple cycles of transcription. Chaperones play roles in DNA packing at replication forks: 1‒disruption; 2‒transfer (recycling histones); 3‒transfer (newly synthesized histones); 4 and 5‒deposition/de novo assembly, are illustrated. However, in aged cells, the lengthened cell cycle reduces replication-dependent nucleosome assembly accompanied by incorporation of newly synthesized histones. Instead, other combinatory histone chaperones deposit histone variants through replication-independent nucleosome assembly, leading to ectopically and fuzzily positioned nucleosomes, causing cryptic transcription. Histones H2A, H2B, and H2A.Z are observed in histone locus bodies. H2A and H2AZ in lipid droplets may contribute to replication-independent nucleosome assembly