A cell is eukaryotic if it keeps its DNA inside a membrane-enclosed nucleus. That single feature, a true nucleus, is the defining boundary between eukaryotic cells (animals, plants, fungi, protists) and prokaryotic cells (bacteria, archaea), which let their DNA float freely in the cytoplasm. But the nucleus is just the headline. Eukaryotic cells differ from prokaryotes in how they organize DNA, generate energy, divide, and maintain internal structure.
The Nucleus Sets Eukaryotes Apart
The nucleus is surrounded by a double-layered membrane called the nuclear envelope. The outer layer is physically continuous with the endoplasmic reticulum, the cell’s protein- and lipid-manufacturing network. Scattered across this envelope are nuclear pore complexes, each about 100 nanometers wide, built from roughly 30 different proteins. These pores act as selective gatekeepers: small molecules pass through freely, while proteins and RNA are actively shuttled in or out by transport receptors. This controlled exchange lets the cell separate the job of reading DNA from the job of building proteins, something prokaryotes cannot do.
DNA Packaged Into Linear Chromosomes
Prokaryotic cells typically carry a single circular chromosome. Eukaryotic cells carry multiple linear chromosomes, and those chromosomes are elaborately packaged. The DNA wraps around small, positively charged proteins called histones. Five major types (H1, H2A, H2B, H3, and H4) are found across virtually all eukaryotic species. A stretch of 146 DNA base pairs coils 1.65 times around a core of eight histone molecules, forming a bead-like unit called a nucleosome.
These nucleosomes then coil and fold further, condensing meters of DNA into a nucleus just a few micrometers across. Chromatin, the name for this DNA-protein complex, contains roughly twice as much protein as DNA by mass. This layered packaging does more than save space. It also gives the cell a powerful way to control which genes are turned on or off, because tightly wound DNA is harder for the cell’s machinery to read than loosely wound DNA.
Membrane-Bound Organelles
The nucleus is the most prominent membrane-enclosed compartment, but eukaryotic cells contain many others. Each one creates a specialized chemical environment for a specific job.
- Mitochondria generate most of the cell’s energy through aerobic respiration.
- Endoplasmic reticulum (ER) comes in two forms: rough ER (studded with ribosomes) synthesizes proteins, while smooth ER produces lipids.
- Golgi apparatus receives proteins and lipids from the ER, processes them, and sorts them for delivery to other parts of the cell or for export.
- Lysosomes contain digestive enzymes that break down worn-out cell parts, captured bacteria, or food particles.
- Peroxisomes neutralize toxic byproducts of metabolism, including hydrogen peroxide.
- Chloroplasts (in plants, algae, and some protists) carry out photosynthesis.
Prokaryotic cells have none of these compartments. Their chemical reactions happen in the cytoplasm or at the cell membrane, which limits how many distinct processes can run simultaneously. The compartmentalization inside eukaryotic cells lets incompatible reactions occur side by side without interfering with each other.
A Complex Internal Skeleton
Eukaryotic cells maintain their shape and move materials around using a cytoskeleton made of three types of protein filaments: actin filaments, intermediate filaments, and microtubules. Actin filaments support the cell’s outer edges and drive changes in shape, like the crawling movement of white blood cells. Intermediate filaments provide mechanical strength, acting like internal cables. Microtubules serve as tracks for transporting organelles and play a central role in cell division by pulling chromosomes apart.
Prokaryotes have simpler structural proteins that serve some overlapping functions, but nothing approaching the versatility and organization of the eukaryotic cytoskeleton.
Larger Ribosomes
All cells use ribosomes to build proteins, but eukaryotic ribosomes are noticeably bigger. They measure 80S (a unit reflecting how fast they settle in a centrifuge), with a 40S small subunit and a 60S large subunit. Their total molecular mass is about 4.3 million daltons. Bacterial ribosomes, by comparison, are 70S with a mass of roughly 2.3 million daltons. The extra size reflects additional RNA and protein components that give eukaryotic ribosomes more sophisticated regulation of protein production.
Cell Division Through Mitosis
Prokaryotes reproduce by binary fission, a straightforward process: the circular chromosome copies itself, the two copies move to opposite ends of the cell, and the cell pinches in half. There is no spindle apparatus, no condensation of chromosomes into visible structures.
Eukaryotic cells divide through mitosis, a more elaborate process involving four distinct stages. The chromosomes condense tightly, a spindle made of microtubules forms, and each chromosome attaches to spindle fibers that pull identical copies to opposite poles. Organelles are also duplicated beforehand so each daughter cell gets a full set. This complexity is necessary because eukaryotic cells have far more DNA spread across multiple chromosomes, and errors in sorting would be catastrophic.
How Eukaryotic Cells Evolved
The leading explanation for how eukaryotic cells acquired their complexity is endosymbiotic theory. Around two billion years ago, a large host cell (likely an archaeon) engulfed a smaller bacterium capable of using oxygen to produce energy. Instead of being digested, that bacterium survived inside the host and eventually became the mitochondrion. A similar event later gave rise to chloroplasts, when a cell that already had mitochondria engulfed a photosynthetic cyanobacterium.
Several lines of evidence support this. Mitochondria and chloroplasts both carry their own DNA, separate from the nucleus. They are surrounded by double membranes, consistent with being engulfed by another cell. And molecular studies have confirmed that only cells possessing mitochondria ever achieved the energy output needed for eukaryotic-level complexity, which is why no true intermediate between prokaryotes and eukaryotes has ever been found. Over time, many genes from the engulfed organisms were transferred to the host cell’s nuclear chromosomes, binding the two partners into a single organism.
Exceptions That Prove the Rule
Not every eukaryotic cell has a nucleus at every stage of its life. Mature red blood cells in mammals eject their nucleus during development, freeing up space to carry more hemoglobin. Platelets, which help blood clot, are also nucleus-free fragments. These cells are still eukaryotic because they originate from nucleated precursor cells and belong to eukaryotic organisms. They simply shed the nucleus as a final step of specialization.
Cell size is another area with more variation than textbooks sometimes suggest. Eukaryotic cells are generally much larger than prokaryotes, but their diameter can range from less than a micrometer (some yeasts and parasites) to several centimeters (like the yolk of a bird egg, which is technically a single cell). The typical animal or plant cell falls somewhere between 10 and 100 micrometers, roughly 10 times the width of a typical bacterium.