This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The human genome is about two meters (6.6 feet) long, yet it is folded inside a cell nucleus only about 10 micrometers in diameter. To fit into this tiny space, DNA is wrapped around histone proteins to form nucleosomes, which are further organized into chromatin.
For decades, chromatin has often been described in two simple forms: euchromatin, which is active, open and accessible, and heterochromatin, which is more compact and repressed. However, a new study from an international team led by Kazuhiro Maeshima, a professor at the National Institute of Genetics, ROIS (Research Organization of Information and Systems) and SOKENDAI, has challenged this simple textbook view. The researchers demonstrated that euchromatin in living human cells is not merely open and loose but forms dynamic condensed domains.
This domain organization helps prevent the mixing of neighboring domains. The team further found that cohesin, a ring-shaped protein complex best known for organizing genome architecture, prevents local mixing between these condensed euchromatic domains for proper gene regulation in living human cells. The study was published in Nature Genetics on Sept. 8, 2026.
The cohesin complex is widely known for forming chromatin loops and contributing to genome organization. In this study, the researchers asked whether cohesin also controls the physical properties of euchromatic domains at the level of individual nucleosomes. To address this, the team combined single-nucleosome imaging and tracking in living human cells with super-resolution 3D-structured illumination microscopy (3D-SIM).
Single-nucleosome imaging allowed the researchers to follow the movement of individual nucleosomes, while 3D-SIM enabled them to visualize the euchromatic domains at about 100 nm resolution. The researchers found that removing cohesin increased the mobility of nucleosomes within euchromatic domains, making them more fluidlike. Surprisingly, this increase in fluidity occurred without altering overall chromatin compaction.
In other words, euchromatic domains did not simply open up or decondense globally. Instead, cohesin loss caused neighboring condensed domains to mix locally. "This was an important point for us," said Maeshima.
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