A nucleosome is the basic packaging unit of DNA inside your cells. It consists of a segment of DNA wrapped around a cluster of eight proteins called histones, forming a spool-like structure about 11 nanometers wide. Every cell in your body contains roughly two meters of DNA, and nucleosomes are what make it possible to fit all of that into a nucleus just a few thousandths of a millimeter across.
How a Nucleosome Is Built
The core of a nucleosome is a set of eight histone proteins: two copies each of four types called H2A, H2B, H3, and H4. Together, these eight proteins form a disc-shaped structure known as the histone octamer. About 146 to 147 base pairs of DNA wrap around this protein disc roughly 1.65 times, like thread around a spool. The entire core particle weighs about 200 kilodaltons and has a diameter of roughly 10 nanometers (100 angstroms), with a height that varies from about 2.5 nanometers at the center to 6 nanometers at the widest point.
Between one nucleosome and the next sits a short stretch called linker DNA, typically 20 to 80 base pairs long. A fifth type of histone, called H1, sits at the spot where DNA enters and exits the nucleosome core. H1 clamps the DNA in place, stabilizing the whole structure and influencing how much space separates neighboring nucleosomes. When you include H1 and about 165 base pairs of DNA, the complete unit is sometimes called a chromatosome.
Why DNA Needs Nucleosomes
Bare DNA is a thread about 2 nanometers wide. Wrapping it into nucleosomes achieves roughly a sevenfold linear compaction right away. That alone isn’t enough to fit two meters of DNA into a cell nucleus, but it’s the critical first step. The string of nucleosomes, often described as looking like “beads on a string,” is called the 10-nanometer fiber.
For decades, textbooks described a next level of folding: the 10-nanometer fiber coiling into a thicker 30-nanometer fiber, first observed in the 1970s using electron microscopy on purified chromatin. More recent work using cryo-electron microscopy and X-ray scattering inside actual cells tells a different story. Chromatin in living cells appears to consist of irregularly folded 10-nanometer fibers rather than neat 30-nanometer coils. The 30-nanometer fiber may exist only under artificial lab conditions, not inside real nuclei or chromosomes.
How Cells Control Access to DNA
Nucleosomes do more than just package DNA. They also regulate which genes get read. A gene buried under a tightly packed nucleosome is effectively silenced because the cell’s reading machinery can’t reach it. To activate that gene, the cell has to move, loosen, or remove the nucleosome.
Specialized protein machines called chromatin remodeling complexes handle this job. They burn ATP (the cell’s energy currency) to physically slide nucleosomes along the DNA strand, swap out individual histones, or eject nucleosomes entirely. Different remodeling complexes use slightly different strategies. Some peel DNA off the histone surface and create a bulge or loop that travels around the nucleosome, effectively repositioning it. Others simply push the whole nucleosome far enough along the strand that the target DNA segment ends up in the exposed linker region between nucleosomes.
Chemical Tags That Change Nucleosome Behavior
Each histone protein has a flexible tail that sticks out from the nucleosome core. Cells attach small chemical groups to these tails, and the specific combination of tags, sometimes called the “histone code,” profoundly affects whether nearby genes are turned on or off.
Acetylation is one of the most studied modifications. Adding an acetyl group to a histone tail neutralizes some of its positive electrical charge, weakening the grip between the histone and the negatively charged DNA. The result is a looser, more open chromatin structure that allows genes to be read. When enzymes remove those acetyl groups, histones bind DNA more tightly, chromatin compacts, and gene activity drops.
Methylation works differently. It’s a more stable, longer-lasting mark, making it well suited for carrying epigenetic information that persists across cell divisions. Depending on which histone and which specific spot gets methylated, the effect can either activate or silence a gene. Phosphorylation, another common modification, reduces the attraction between histones and DNA, loosening chromatin structure and helping transcription factors reach the DNA. Ubiquitination can go the other way: adding a ubiquitin tag to histone H2A promotes tighter binding with linker histone H1 and stabilizes the nucleosome, making it harder for the cell to strip DNA off the surface.
These modifications work in combination. A nucleosome might carry acetylation on one tail and methylation on another, and the net effect depends on the full pattern. This layered system gives cells fine-grained control over gene activity without changing the underlying DNA sequence.
When Nucleosomes Were Discovered
The nucleosome model was proposed in 1974 by Roger Kornberg, who later won the Nobel Prize. His key insight, published in Science, was that chromatin has a repeating unit made of eight histone molecules and about 200 base pairs of DNA. A few years later, electron microscopy images revealed the beads-on-a-string appearance, confirming the model. In 1997, a team led by Timothy Richmond published the first high-resolution crystal structure of the nucleosome core particle at 2.8 angstroms, showing in atomic detail exactly how the DNA superhelix wraps around the histone octamer.
Nucleosomes as Disease Markers
When cells die, their contents spill into the bloodstream, including intact nucleosomes. In healthy people, circulating nucleosome levels stay low. In people with cancer, those levels rise, sometimes dramatically. Patients with breast, lung, prostate, and colorectal cancer consistently show elevated circulating nucleosomes compared to healthy controls, particularly in advanced stages. The fraction of tumor-derived nucleosomes in a person’s blood can range from as little as 3% to as much as 93% of total plasma nucleosomes.
Measuring circulating nucleosome levels alone isn’t enough to diagnose a specific cancer, since any condition that causes widespread cell death can elevate them. But analyzing the histone modification patterns on those circulating nucleosomes, combined with other tumor markers, can improve diagnostic accuracy. During chemotherapy or radiation, tracking nucleosome levels in the blood offers a real-time window into whether treatment is working. A spike in circulating nucleosomes after treatment suggests massive tumor cell death, a sign the therapy is hitting its target. If levels stay flat, the tumor may not be responding. In patients who achieve remission, circulating nucleosome levels drop quickly, while persistently high or rising levels tend to signal disease progression.