Mitochondria are tiny parts inside every cell. They make the energy that cells need to work. Because they do this job, they keep a small set of genes called mitochondrial DNA, or mtDNA.
Each cell contains hundreds to thousands of copies of mtDNA. These copies are packed together in tight bundles called nucleoids. Scientists have noticed that nucleoids are arranged in a regular pattern inside mitochondria. This pattern helps the DNA be shared correctly when a cell divides and keeps the genes working evenly.
If mitochondria or their DNA stop working right, the results can be serious. Problems with mtDNA are linked to liver failure, brain disorders, and age‑related diseases such as Alzheimer’s and Parkinson’s.
A Long‑Standing Mystery
Because mtDNA is so important, researchers have tried for years to learn how cells keep nucleoids spaced so evenly. The answer was not clear.
“Ideas about mitochondrial fusion, fission, or molecular tethers don’t fully explain the spacing,” says Suliana Manley, a professor at EPFL’s Laboratory of Experimental Biophysics.
Manley and post‑doctoral fellow Juan Landoni discovered the missing piece: a process called mitochondrial pearling. This process had been mentioned before but was largely ignored.
What Is Mitochondrial Pearling?
During pearling, a mitochondrion temporarily reshapes into a string of beads. While it looks like a necklace, the mtDNA clusters move apart and settle into the new “beads.” This spreads the nucleoids evenly and keeps their regular spacing.
Seeing Pearling in Live Cells
To watch this happen, the team used several high‑tech imaging tools: super‑resolution microscopy, correlated light‑and‑electron microscopy, and phase‑contrast microscopy. These methods let them follow single nucleoids and capture fast shape changes.
How Pearling Works
Live‑cell videos showed that pearling can happen several times each minute. The mitochondrion briefly forms evenly spaced constrictions, called “pearls.” The distance between pearls matches the usual distance between nucleoids.
Most pearls contain a nucleoid near their center, though some pearls appear without any DNA. Larger groups of nucleoids often break into smaller pieces that settle into neighboring pearls. When the mitochondrion returns to its normal tube shape, the nucleoids stay apart, preserving the even pattern.
What Triggers Pearling?
The researchers tested genes and chemicals to find the drivers. They learned that calcium entering the mitochondrion can start pearling. Inside the organelle, special membrane structures also help keep the nucleoids separated.
If calcium influx or those membrane helpers are blocked, nucleoids tend to clump together instead of staying evenly spaced.
Why This Matters
Back in 1915, Margaret Reed Lewis first drew mitochondria looking like beads. For a long time, scientists thought it was just a stress‑related oddity. Today, we see that pearling is a conserved, energy‑saving way to distribute mitochondrial DNA.
Understanding this simple physical process gives new insight into diseases caused by mtDNA problems. It could eventually lead to better ways to treat conditions linked to faulty mitochondria.