Dynamics of chromatin structures plays a critical role in transcriptional regulation and all other DNA related biological processes. Previously, we reported the 11 ？ resolution cryo-electron microscopy (cryo-EM) structures of 30 nm chromatin fibers reconstituted in the presence of linker histone H1, which reveals a left-handed double helix twisted by the repeating tetra-nucleosomal structural units. Recently, we have determined the 3.6 ？ resolution cryo-electron microscopy (cryo-EM) structures of 30 nm chromatin fibers with linker histone H5, uncovering that asymmetries and polarities of nucleosomes play important roles in the folding of chromatin fibers. In addition, we also demonstrated that formation of 30-nm chromatin fibers greatly facilitates the faithful propagation of H2AK119ub1 and H3K27me3 during cell divisions.