A Molecular View of Genome Packaging in Human Cells

How do two meters of DNA fit inside the cell nucleus? Researchers used cryo-electron tomography to map individual nucleosomes inside human cells.

Researchers at the Max Planck Institute of Biophysics, in collaboration with Heidelberg University, captured the molecular architecture of DNA packaging inside human cells. Using cryo-electron tomography and computational simulations, the team identified individual nucleosomes in immune cells, revealing a densely packed but irregular chromatin arrangement at the edge of the nucleus. The study opens the door to connecting structural biology and genomics to study DNA organization.

Every human cell faces an extraordinary packaging problem. Around two meters of DNA must fit inside a nucleus only a few micrometers wide, smaller than a speck of dust, while staying accessible enough to be read, copied, repaired, and regulated on demand. This packaging is equivalent to fitting a 100 meter ball of yarn into a grain of salt, without losing track of its ends. Nature’s solution is chromatin: DNA wrapped around histone protein complexes, forming repeating units called nucleosomes. Scientists have studied nucleosomes for decades, but mostly outside their usual setting, isolated from the crowded environment of real cells. How chromatin looks and behaves inside intact human cells has been far more difficult to observe.

In a new study published in Nature Communications, researchers from the Max Planck Institute of Biophysics, in collaboration with Heidelberg University, have captured a detailed molecular view of chromatin directly inside its native cellular environment. By combining cryo-electron tomography with computer simulations, the team mapped individual nucleosomes inside T cells, a type of human immune cell, revealing their previously hidden architecture at the edge of the cell’s nucleus.

Chromatin in its natural habitat

Chromatin is often illustrated as neatly ordered nucleosome beads threaded on a DNA string. But inside cells, things are more chaotic. Chromatin is crowded, dynamic, chemically tagged, and surrounded by countless other proteins. It is this chaos that makes it challenging to tell whether the textbook picture of ordered chromatin fibers truly reflects the organization of DNA inside living cells.

To find out, the research team used cryo-electron tomography, a technique that flash-freezes cells and images them in 3D, preserving their internal structures almost intact. They focused on the nuclear periphery of resting human T cells, a region known to be packed with a dense form of chromatin called heterochromatin. Using advanced computational tools, the researchers saw individual nucleosomes directly in their tomograms and mapped exactly where they were and how they were oriented.

Jan Philipp Kreysing, Sergio Cruz-León, and Johannes Betz, authors of the study from the Department of Molecular Sociology and the Department of Theoretical Biophysics at the Max Planck Institute of Biophysics, share their excitement about the new possibilities to study nucleosomes directly inside cells:

This is exciting because we can now begin to study chromatin architecture inside cells at the level of individual nucleosomes. Instead of inferring organization only from averaged or indirect methods, we can observe local molecular arrangements in their native environment.

From single nucleosomes to a chromatin map

The team didn’t only spot nucleosomes, they also resolved their structure to sub-nanometer resolution. Such fine detail was enough to see individual features of the DNA and its surrounding proteins, even inside the crowded cell nucleus. They then used physics-based computational modeling to trace estimated paths of DNA linkers connecting neighboring nucleosomes. This allowed them to see beyond single nucleosome “beads”, essentially reconstructing short stretches of the whole 3D chromatin structure.

Their results revealed that chromatin at the edge of the nucleus is densely packed, but it doesn’t form the long-debated 30-nanometer fiber, a tight and regular model of chromatin organization. Instead, the team found a more irregular, looser, elongated structure about 37 nanometers wide. This arrangement contains ordered regions, where nucleosomes stack neatly together, mixed with stretches of variable linker DNA spacing and nucleosome orientation. In other words, this confirms tight DNA packing, but far from the classical, tidy chromatin models.

Structure meets genomics

The work brings together the two traditional ways to study DNA: structural biology, which zooms in on individual molecules, and genomics, which maps DNA organization across the genome. By measuring how nucleosomes are spaced and arranged directly inside cells, the study provides a new structural foundation for understanding how genome packaging relates to gene regulation. The authors emphasize the relevance of this methodology:

Our study shows that in situ structural biology can reveal chromatin organization at a scale that was previously difficult to access. This opens the door to comparing different cell states, perturbations, or disease-relevant conditions directly inside the nucleus.

Looking ahead, improvements in cryo-electro tomography, image processing, and computational modeling could allow researchers to capture active chromatin regions and regulatory machinery in even greater detail. In the long term, this kind of research could explain how DNA organization shapes gene activation, cell differentiation, and genome function.