Geometry doing the work that molecules couldn't fully explain.
Inside every living cell, tucked within the mitochondria, sits a small and ancient genome — a remnant of the bacterial ancestor that merged with our cells more than a billion years ago. That genome, mitochondrial DNA, is copied hundreds to thousands of times per cell and bundled into clusters called nucleoids. For decades, biologists noticed something striking about those clusters: they are spaced with unusual regularity along the length of mitochondria, like evenly placed beads. Nobody could fully explain why.
The spacing matters enormously. When cells divide, mitochondrial DNA has to be distributed reliably to daughter cells. Genes encoded in that DNA need to be expressed uniformly along the mitochondrion's length. When either process goes wrong, the consequences can be severe — liver failure, encephalopathy, and a range of neurodegenerative conditions including Alzheimer's and Parkinson's disease all carry links to mitochondrial dysfunction. The stakes of understanding nucleoid spacing, in other words, are not merely academic.
Previous attempts to explain the spacing pointed to familiar mitochondrial behaviors: the organelles constantly fuse and split, and molecular tethers can anchor structures in place. But those explanations kept running into a stubborn problem. Disrupt fusion, disrupt fission, disrupt the tethers — and the nucleoids still end up evenly spaced. Something else was doing the work.
A team at EPFL's Laboratory of Experimental Biophysics, led by professor Suliana Manley and postdoctoral fellow Juan Landoni, has now identified what that something is. The answer turns out to be a phenomenon that scientists had largely written off as a curiosity: mitochondrial pearling. During pearling, a mitochondrion temporarily transforms from its usual tube-like shape into a chain of rounded bulges separated by narrow constrictions — the beads-on-a-string appearance that gives the process its name. The team's work, published in early April 2026, shows that this transient shape change is not a stress artifact but a genuine, conserved mechanism for redistributing the mitochondrial genome.
To watch it happen, the researchers assembled an unusually broad toolkit of imaging methods. Super-resolution microscopy let them resolve fine structural details. Correlated light and electron microscopy gave them complementary views of the same events. Gentler techniques like phase contrast microscopy allowed them to observe living cells without damaging them. Together, these approaches let the team track individual nucleoids in real time and capture the rapid structural changes unfolding inside mitochondria.
What they saw was both elegant and surprisingly frequent. Pearling events occur several times per minute in active cells. As the constrictions form, the spacing between adjacent pearls closely mirrors the typical distance between nucleoids. Most pearls contain a nucleoid near their center. Crucially, when a larger cluster of nucleoids sits inside a pearling mitochondrion, the cluster tends to split — each fragment settling into its own pearl. When the mitochondrion relaxes back into its tubular form, those fragments stay separated. The regular spacing is established and maintained through this repeated, transient reshaping.
The team also traced the regulatory machinery behind the process. Calcium flowing into the mitochondrion can trigger pearling. Internal membrane structures help hold the redistributed nucleoids in place once pearling subsides. Interfere with either the calcium signal or those internal structures, and the nucleoids clump together rather than spreading out.
What makes the finding historically resonant is that pearling is not new to science — it is simply newly understood. The cell biologist Margaret Reed Lewis sketched mitochondrial pearling in 1915, more than a century ago. For most of the intervening decades, the phenomenon was treated as a sign of cellular stress, something that happened to sick or damaged cells rather than a routine feature of healthy ones. Landoni described it as a mechanism that has been overlooked for over a hundred years, now emerging as a conserved and energy-efficient way for cells to distribute their mitochondrial genome.
The broader implication is that cells do not rely solely on molecular motors and protein scaffolds to organize their interiors. Physical processes — the mechanics of membrane tension and shape change — can do real biological work. For researchers studying diseases tied to mitochondrial DNA, the discovery opens a new set of questions: what happens to pearling in diseased tissue, and could restoring or modulating it become a therapeutic target? Those questions are now, at least, worth asking.
Notable Quotes
Proposed mechanisms related to mitochondrial fusion, fission, or molecular tethering cannot explain the spacing, since it is maintained even when they are disrupted.— Suliana Manley, professor, Laboratory of Experimental Biophysics, EPFL
Over a century after it was first observed, pearling is emerging as an elegantly conserved mechanism at the heart of mitochondrial biology — a biophysical process offering a simple, energy-efficient means to distribute the mitochondrial genome.— Juan Landoni, postdoctoral fellow, Laboratory of Experimental Biophysics, EPFL
The Hearth Conversation Another angle on the story
Why had nobody figured this out before? Mitochondria have been studied for a long time.
They had, but pearling kept getting dismissed. When you see a mitochondrion bulging into beads, the instinct is to assume the cell is stressed or dying. It took careful live-cell imaging to show it happening routinely, several times a minute, in healthy cells.
What made the spacing problem so hard to crack?
The usual suspects — fusion, fission, molecular tethers — all failed the test. You could knock them out genetically and the nucleoids still ended up evenly spaced. That meant something else was responsible, and nobody had a good candidate.
So pearling physically separates the DNA clusters?
Exactly. When the constrictions form, a large nucleoid cluster gets pinched apart, one fragment per pearl. When the mitochondrion relaxes, those fragments stay where they landed. It's almost mechanical — geometry doing the work that molecules couldn't fully explain.
What role does calcium play?
Calcium entering the mitochondrion can trigger the pearling event itself. It's one of the signals that sets the whole reshaping process in motion. Disrupt that signal and the nucleoids start clumping instead of spreading.
Is this something unique to certain cell types?
The evidence points to it being conserved — meaning it shows up broadly rather than in one specialized cell type. That's part of what makes it significant. It looks like a general feature of mitochondrial biology, not an edge case.
Margaret Reed Lewis drew this in 1915. What does it mean that it took a century to take seriously?
It's a reminder that observation and interpretation are different things. She saw the shape change clearly. But the field's assumption — that pearling meant damage — was sticky enough to delay the right question by over a hundred years.
What's the disease angle here?
Mitochondrial DNA problems are linked to conditions ranging from liver failure to Parkinson's. If pearling is how cells maintain healthy nucleoid distribution, then understanding what disrupts pearling in disease tissue could point toward new ways to intervene.