Nanoscale analysis of human G1 and metaphase chromatin in situ

The structure of chromatin at the nucleosome level inside cells is mysterious. Here we present in situ electron cryotomography analyses of chromatin in both G1 and metaphase RPE-1 cells. G1 nucleosomes are concentrated in globular chromatin domains and metaphase nucleosomes are concentrated in the chromatids. Classification analysis reveals that canonical mononucleosomes, ordered stacked dinucleosomes, and mononucleosomes with a disordered gyre-proximal density are abundant in both cell-cycle states. Class averages that have more than two stacked nucleosomes or that have side-by-side dinucleosomes are not detected, suggesting that groups of more than two nucleosomes are heterogeneous. Large multi-megadalton structures are abundant in G1 nucleoplasm, but not found in G1 chromatin domains and metaphase chromatin. The macromolecular phenotypes studied here represent a starting point for the comparative analysis of compaction in normal and unhealthy human cells, other cell-cycle states, other organisms, and in vitro chromatin assemblies.