Bacteria and archaea, the two domains of prokaryotic life, are the organisms most commonly associated with a single circular chromosome. Most bacterial species carry their entire genome on one circular DNA molecule, and many archaea do the same. This is fundamentally different from eukaryotes (animals, plants, fungi), which package their DNA across multiple linear chromosomes inside a membrane-bound nucleus.
Bacteria: The Classic Example
The vast majority of bacteria carry a single circular chromosome housed in a region of the cell called the nucleoid. Unlike a eukaryotic nucleus, the nucleoid has no surrounding membrane. The chromosome sits directly in the cell’s interior, compacted by specialized proteins and tightly coiled through a process called supercoiling. If you stretched a typical bacterial chromosome out to its full length, it would measure nearly one millimeter, roughly 1,000 times the length of the cell it fits inside.
Escherichia coli, the most studied bacterium in biology, is the textbook example. Its single circular chromosome is about 4.6 million base pairs long. Other well-known species with one circular chromosome include Bacillus subtilis, Staphylococcus aureus, and Mycobacterium tuberculosis. Genome sizes across bacteria range widely, from under 1 million base pairs in some parasitic species to nearly 10 million in free-living soil bacteria like Solibacter usitatus (9.97 million base pairs) and Myxococcus xanthus (9.1 million base pairs).
Bacterial chromosomes replicate from a single starting point called the origin of replication (oriC). A protein called DnaA recognizes this site and opens the DNA double helix so copying can begin. Replication proceeds in both directions around the circle, with two replication forks meeting on the opposite side. This “one origin, one round” system is efficient for organisms that need to divide quickly, sometimes in as little as 20 minutes under ideal conditions. Bacteria can even start a new round of replication before the previous one finishes, allowing them to temporarily run several chromosome cycles in parallel during rapid growth.
Archaea: Similar Shape, Different Machinery
Archaea also possess simple, circular chromosomes, and superficially their genomes look a lot like bacterial ones. The key differences are in the molecular machinery. Where bacteria use DnaA to kick off replication, archaea use a protein called Orc1/Cdc6, which is related to proteins found in eukaryotic cells. This is one of many features that place archaea closer to eukaryotes on the evolutionary tree, despite their prokaryotic cell structure.
Another important distinction is replication origins. While bacteria have a single origin per chromosome, archaea are more variable. Pyrococcus, a heat-loving archaeon found near deep-sea hydrothermal vents, uses just one origin, much like bacteria. But this turns out to be the exception among archaea. Members of the genus Sulfolobus have three replication origins, Pyrobaculum calidifontis has four, and Haloferax species also use three. So while the chromosome is still circular, the replication strategy is more complex than what you see in most bacteria.
Circular DNA Inside Eukaryotic Cells
Eukaryotes, the domain that includes humans, animals, plants, and fungi, organize their nuclear genomes on multiple linear chromosomes. But they do contain circular DNA in two types of organelles: mitochondria and chloroplasts.
Mitochondria, the energy-producing structures found in nearly all eukaryotic cells, carry small circular chromosomes of their own. In humans, mitochondrial DNA is about 16,500 base pairs, encoding just 37 genes. Chloroplasts, found in plants and algae, also contain circular DNA, typically around 120,000 to 160,000 base pairs. Both organelles are thought to descend from ancient bacteria that were engulfed by early eukaryotic cells billions of years ago. Their circular chromosomes are a remnant of that bacterial ancestry. Over evolutionary time, most of the original genes migrated to the host cell’s nucleus, leaving these organelles with stripped-down genomes.
It’s worth noting that researchers have found the actual DNA molecules inside chloroplasts and mitochondria aren’t always neat circles. Many exist as linear or branched structures, likely intermediates of replication and recombination. The genome itself maps as a circle, but the physical molecules in the organelle are more varied than the textbook picture suggests.
Exceptions to the Single Circular Rule
Not every prokaryote follows the standard pattern. Several well-known bacteria break the rule in one of two ways: by having linear chromosomes or by having more than one chromosome.
Borrelia burgdorferi, the bacterium that causes Lyme disease, carries a linear chromosome rather than a circular one. Related species like Borrelia afzelii and Borrelia garinii do the same. Agrobacterium tumefaciens, a soil bacterium widely used in plant genetic engineering, has two chromosomes: one circular (about 3 million base pairs) and one linear (about 2.1 million base pairs), along with two smaller plasmids.
Vibrio cholerae, the cause of cholera, is one of the best-known examples of a bacterium with two circular chromosomes. Rhodobacter sphaeroides and several other species also maintain multiple chromosomes. These cases are genuine exceptions, though. The single circular chromosome remains the dominant arrangement across the bacterial world.
Why Circular Chromosomes Persist
Circular chromosomes offer several practical advantages for small, fast-dividing cells. Having no ends eliminates a problem that linear chromosomes face: the “end-replication problem,” where DNA is lost from chromosome tips with each round of copying. Eukaryotes solve this with telomeres, protective caps that are gradually shortened and must be rebuilt. Circular chromosomes sidestep the issue entirely.
The single-origin replication system also pairs well with a circular layout. It allows the chromosome cycle to be tightly linked to cell division, or temporarily decoupled from it when conditions demand. During rapid growth, bacteria can initiate multiple overlapping rounds of replication on the same chromosome. During stress, they can pause division while maintaining the chromosome. This flexibility, enabled by the circular architecture and its simple replication logic, likely explains why the arrangement has been so successful across billions of years of prokaryotic evolution.