Circular DNA: Structure, Role, and Impact in Health

Circular DNA is not one thing but a broad family of ring-shaped DNA molecules that show up across virtually every domain of life, from the tiny genomes inside your mitochondria to the rogue genetic elements that help tumors resist chemotherapy. Some forms are essential for normal cell function, others are linked to aging, and still others are being engineered as tools for gene therapy. The biology of circular DNA has expanded rapidly in the last decade, reshaping how researchers think about cancer, infectious disease, and even what counts as “normal” in a healthy human genome.

The Many Faces of Circular DNA

The phrase “circular DNA” covers several distinct molecular species, and lumping them together causes confusion. The most familiar is mitochondrial DNA: the small, circular, double-stranded genome carried inside the energy-producing compartments of nearly every human cell. Mammalian mitochondria maintain multiple copies of this genome, and a dedicated set of replication machinery keeps it going.¹ Mutations in mitochondrial DNA underlie a range of inherited metabolic diseases, and its circular structure has been recognized for decades.

Then there is extrachromosomal circular DNA, or eccDNA, a catch-all term for circular DNA fragments that originate from chromosomes but float freely in the nucleus. These come in a dizzying range of sizes. The smaller varieties, sometimes called microDNA, can be just a few hundred base pairs long. At the other end, large cancer-associated circles called ecDNA can span millions of base pairs and carry entire oncogenes. The terminology is messy because different research groups coined different names over the years, and only recently has the field tried to standardize.² For clarity, this article uses “eccDNA” for the general class and “ecDNA” specifically for the large, cancer-relevant circles.

Beyond the nucleus and mitochondria, bacteria carry plasmids, which are small circular DNA molecules that replicate independently and often shuttle antibiotic-resistance genes between species. And certain viruses, like hepatitis B, maintain their own circular DNA inside infected cells. Each of these forms has different biology, different health consequences, and different research trajectories.

Circular DNA in Healthy Human Tissue

For a long time, eccDNA was viewed mainly as a curiosity of cancer cells. That changed when researchers began looking for it systematically in normal tissue. A study that purified and sequenced eccDNA from muscle and blood samples of 16 healthy men detected roughly 100,000 unique eccDNA types from about 16 million nuclei.³ That is a striking number. About half of these circles carried genes or gene fragments, and most were smaller than 25,000 base pairs. Gene-rich chromosomes contributed more eccDNA per unit of length, and the most actively transcribed gene in muscle tissue, titin, generated the most eccDNA of any single gene.³

What these circles are doing in healthy cells is still an open question. One possibility is that they are simply byproducts of normal DNA repair and replication: snipped-out loops that accumulate harmlessly. Another is that they influence gene activity by altering local copy numbers, essentially giving certain genes an extra dose. The sheer abundance of eccDNA in non-cancerous tissue suggests it is not just noise, but pinning down a clear physiological role remains a work in progress.

How ecDNA Drives Cancer

The health impact of circular DNA is most dramatic in cancer. Large ecDNA molecules can carry amplified copies of oncogenes, and because they sit outside chromosomes, they do not follow the usual rules of inheritance during cell division. When a cell divides, chromosomes are carefully split so each daughter cell gets one copy. ecDNA, by contrast, segregates unevenly, with some daughter cells receiving many copies and others receiving few. Live-cell imaging has confirmed this uneven distribution directly.⁴ The result is that a single tumor can contain cells with wildly different numbers of oncogene copies, creating a reservoir of genetic diversity that natural selection within the tumor can act on. If a drug targets that oncogene, cells that happen to carry fewer copies may survive, and cells that acquired extra copies through random segregation may thrive.

But copy number alone does not explain ecDNA’s potency. Researchers have found that ecDNA enables changes in gene regulation that go well beyond simply having more copies of a gene. Multiple ecDNA molecules physically cluster together in the nucleus, forming structures called ecDNA hubs, which are micrometer-sized assemblies tethered by proteins.⁵ These hubs allow regulatory elements on one circle to activate genes on a neighboring circle, a kind of cooperative sharing that does not happen with normal chromosomal DNA. Live-cell imaging showed ecDNA hubs forming in more than half of observed cancer cells within 48 hours, and the hubs colocalized with RNA polymerase II, the enzyme responsible for reading DNA into RNA.⁴ Hubs that colocalized with this enzyme were significantly larger than isolated ecDNA, suggesting the clustering itself promotes transcription.

The Epigenetic Landscape of ecDNA

Something unusual is going on with the chemical markings that sit on top of ecDNA. Chromosomal DNA is decorated with epigenetic marks that either encourage or silence gene activity, and these marks follow predictable patterns. ecDNA breaks those patterns. Studies using chromatin analysis have found that ecDNA is enriched with active marks and largely lacks the repressive marks seen on equivalent chromosomal regions.⁶ Gene promoters on ecDNA also show lower levels of DNA methylation compared to the same sequences when they sit on a chromosome, which generally means those promoters are more active.

More recent work has revealed an even stranger feature: ecDNA can carry what researchers call a bivalent epigenetic landscape, with both active and repressive marks coexisting on the same molecule. This combination, distinct from anything seen on chromosomal DNA, appears to let ecDNA remain transcriptionally active even when pushed to the nuclear periphery, a location where chromosomal genes would typically be silenced.⁷ Why ecDNA escapes normal epigenetic regulation is still unclear, but the practical consequence is that oncogenes riding on ecDNA can stay switched on in contexts where chromosomal copies would be turned off.

Circular DNA and Aging

The connection between circular DNA and aging was established in yeast well before the recent cancer research boom. In budding yeast, circles of ribosomal DNA called ERCs accumulate as cells age, and experimental work showed that ERCs are not just a side effect of aging but actually cause it. Yeast mutants lacking the gene SGS1, a counterpart of the human gene involved in Werner syndrome (a premature-aging condition), accumulated ERCs faster and had a shorter lifespan.⁸

More recent work has broadened the picture. Aged yeast do not just pile up ribosomal-DNA circles; they also accumulate other high-copy protein-coding circular DNAs through both random and environmentally driven recombination.⁹ Whether a similar process contributes to human aging remains an open and genuinely interesting question. The finding that healthy human tissues already carry large numbers of eccDNA raises the possibility that these molecules accumulate over a lifetime in ways that could matter, but direct evidence in human cells is still thin. The yeast work remains the strongest proof-of-concept that circular DNA can drive aging rather than merely accompany it.

Viral Circular DNA and Chronic Infection

Some viruses exploit circular DNA to establish infections that last decades. Hepatitis B virus is the most studied example. Once the virus enters a liver cell, it converts its genome into a special form called covalently closed circular DNA, or cccDNA. This molecule sits in the nucleus and serves as the master template for producing all viral RNA transcripts.¹⁰ Critically, cccDNA can persist for decades, even in patients who appear to have recovered from acute infection.¹¹ This persistence is a major reason why hepatitis B is so difficult to cure: antiviral drugs can suppress new virus production but rarely eliminate the cccDNA reservoir hiding inside liver cells.

Other DNA tumor viruses take a related approach. Human papillomavirus, Epstein-Barr virus, and others can maintain their genomes as circular episomes inside the host cell nucleus, replicating alongside the cell’s own DNA without integrating into chromosomes.¹² This episomal persistence can sustain a long-running infection that eventually transforms a normal cell into a cancer cell, highlighting how circular DNA is not just a structural curiosity but a genuine mechanism of disease.

Bacterial Plasmids and the Antibiotic Resistance Crisis

The most consequential circular DNA in public health right now may be bacterial plasmids. These small, self-replicating circular molecules serve as scaffolds on which bacteria assemble arrays of antibiotic resistance genes.¹³ What makes plasmids dangerous is their mobility. Through a process called conjugation, a bacterium can pass a resistance plasmid to a neighbor, even one of a completely different species. The World Health Organization has flagged plasmid-mediated horizontal gene transfer as a major driver of the current antibiotic resistance crisis.¹⁴

A single plasmid can carry resistance to multiple drugs at once, meaning one transfer event can make a previously treatable bacterium resistant to an entire class of antibiotics. The genes arrive pre-assembled by transposable elements and other mobile genetic systems, so the receiving bacterium does not need to evolve resistance on its own. It just needs to pick up the right circle of DNA. This is the mechanism behind many of the multidrug-resistant infections that have become a growing problem in hospitals worldwide.

The Immune System’s Response to Stray DNA

Your cells have sensors designed to detect DNA floating in the wrong place. Under normal conditions, DNA belongs inside the nucleus or mitochondria. When fragments end up in the cytoplasm, the cell treats them as a danger signal, often assuming they came from an invading pathogen. One key detection pathway involves a sensor called cGAS that recognizes double-stranded DNA in the cytoplasm and triggers production of interferons and inflammatory signals. This system evolved primarily to catch viral DNA, but it can also respond to the cell’s own DNA if mitochondrial or nuclear DNA leaks out due to damage or stress.

The relevance to circular DNA is straightforward. eccDNA that escapes the nucleus or mitochondrial DNA released during cell injury can activate the same innate immune pathways. In cancer, this creates a complicated situation: ecDNA-bearing tumor cells may provoke an immune response, but they can also evolve ways to suppress it. In aging, the gradual accumulation of cytoplasmic DNA fragments, potentially including small eccDNA species, may contribute to the chronic low-grade inflammation that characterizes old age. The interplay between circular DNA and the immune system is an active research frontier, with implications for both cancer immunotherapy and age-related inflammatory disease.

Detecting Circular DNA

Studying eccDNA was difficult for decades because standard DNA sequencing methods chew through linear DNA efficiently but struggle to distinguish circular molecules from the vast background of chromosomal fragments. A technique called Circle-Seq changed this. The method works by first denaturing DNA under alkaline conditions, which separates the strands. Circular DNA, being closed, re-anneals efficiently. Any remaining linear DNA is then digested with an enzyme that degrades DNA from free ends, leaving only circles intact. The surviving circular DNA is amplified using rolling-circle amplification before sequencing.² This approach is sensitive enough to detect eccDNA regardless of how many copies are present, and it has uncovered thousands of circular DNA species in organisms ranging from yeast to pigeons to human tissue.

The detection angle matters for clinical applications. Researchers have proposed that ecDNA released by tumor cells into the bloodstream could serve as a component of liquid biopsy, the practice of detecting cancer from a simple blood draw. If tumor-derived circular DNA circulates in blood alongside the linear cell-free DNA already used in some cancer tests, it could provide additional information about tumor origin, drug resistance, and prognosis. This idea is still at the hypothesis stage, but the existence of sensitive detection methods like Circle-Seq makes it technically feasible to test.

Engineered Circular DNA in Gene Therapy

While natural circular DNA can cause trouble, researchers have learned to build synthetic circular DNA for therapeutic purposes. Minicircles are engineered circular expression cassettes produced by cutting away the bacterial backbone of a standard plasmid, including its antibiotic resistance gene, its origin of replication, and inflammatory sequences inherent to bacterial DNA.¹⁵ What remains is a stripped-down circle carrying just the therapeutic gene and its regulatory sequences.

The advantages are practical. Removing the bacterial backbone reduces the immune reaction triggered by the injected DNA, because bacterial sequences contain patterns that human immune sensors recognize as foreign. Minicircles also produce longer-lasting gene expression compared to conventional plasmids. In airway gene-transfer studies, minicircles provided prolonged expression in living animals. When used at the same molecular quantity as standard plasmid DNA, they significantly reduced toxicity and immune activation. When used at the same mass, they significantly boosted the amount of protein the therapeutic gene produced.¹⁶ Minicircles remain a non-viral gene-delivery approach, meaning they avoid the safety concerns associated with viral vectors, though getting them into the right cells efficiently is still a challenge.

Circular DNA in Other Organisms

The story of circular DNA extends well beyond human cells and their pathogens. In plants, the genomes of chloroplasts, the organelles responsible for photosynthesis, were long assumed to be simple circles analogous to mitochondrial DNA. The reality turned out to be more complicated. Replicating chloroplast DNA actually consists of branched, multigenomic structures, and the simple-circle model is something of a textbook oversimplification.¹⁷ Some circular forms exist, but the predominant replicating structures are complex and variable in size. This is a useful reminder that “circular DNA” does not always mean a neat, closed ring; real biology tends to be messier than the diagrams suggest.

In yeast, as noted earlier, circular DNA accumulation is linked to aging. In pathogenic eukaryotes, circular DNA has been identified as a source of genetic variation that allows rapid adaptation, including the development of drug resistance in organisms that cause human disease.⁹ The parallel with cancer ecDNA is hard to miss: in both cases, circular DNA elements provide a fast, flexible route to acquiring new genetic capabilities outside the slow lane of chromosomal mutation. Whether you are a tumor cell resisting chemotherapy or a yeast cell coping with environmental stress, shuffling genes onto circles appears to be a recurring evolutionary strategy.