The main difference between prokaryotes and eukaryotes is the nucleus: eukaryotic cells store their DNA inside a membrane-bound nucleus, while prokaryotic cells do not. This single distinction is so fundamental that it defines the names themselves. “Prokaryote” means “before the nucleus,” and “eukaryote” means “true nucleus.” But that core difference in how DNA is housed ripples outward into nearly every aspect of how these cells are built, how they divide, and how they function.
The Nucleus and Membrane-Bound Organelles
In a eukaryotic cell, DNA sits inside a nucleus enclosed by a double membrane. This creates a separate compartment where genetic information is read and processed before instructions are sent out to the rest of the cell. Prokaryotic cells have no such compartment. Their DNA floats in a region of the cell called the nucleoid, which is not enclosed by any membrane. It’s simply a zone where the DNA tends to cluster.
The membrane theme extends well beyond the nucleus. Eukaryotic cells contain a whole collection of membrane-bound structures: mitochondria (which generate energy), the Golgi apparatus (which packages and ships proteins), and the endoplasmic reticulum (which helps build proteins and lipids). Plant cells add chloroplasts for photosynthesis. Prokaryotic cells lack all of these. Their chemical reactions happen across the cell membrane or directly in the cytoplasm, without dedicated internal compartments.
Size Differences
Prokaryotic cells are dramatically smaller, typically 0.1 to 5.0 micrometers in diameter. Eukaryotic cells range from 10 to 100 micrometers. That means a typical eukaryotic cell could be ten to a thousand times the volume of a prokaryotic one. All those internal organelles help explain why eukaryotic cells can afford to be so much larger: membrane-bound compartments create dedicated spaces for different chemical processes, solving the logistics problem of running a bigger, more complex cell.
How DNA Is Organized
Prokaryotic DNA is usually a single circular chromosome. It’s compact and relatively streamlined, carrying the genes the cell needs without much extra material. Eukaryotic DNA is organized into multiple linear chromosomes, tightly wound around proteins called histones and packaged into complex structures. A human cell, for instance, carries 46 chromosomes. This packaging system allows eukaryotes to manage vastly larger genomes.
How Cells Divide
Prokaryotes reproduce through binary fission, a relatively straightforward process. The circular chromosome copies itself, and the two copies move apart as the cell elongates. Notably, DNA replication and separation happen at the same time rather than in distinct phases. The cell then pinches in half through a contracting ring, producing two identical daughter cells.
Eukaryotic cell division is far more elaborate. Before a cell can split, its DNA must condense into visible chromosomes, and a complex structure called the spindle apparatus must form to pull chromosome copies to opposite ends of the cell. This process, called mitosis, moves through distinct phases: the cell copies its DNA, pauses to prepare, then physically separates the chromosomes before dividing. Eukaryotes also have a second type of division called meiosis, which halves the chromosome number to produce sex cells like sperm and eggs. Prokaryotes have nothing equivalent.
How Genes Become Proteins
Turning a gene into a working protein requires two steps: transcription (copying DNA into a messenger RNA) and translation (reading that RNA to build a protein). In prokaryotes, both steps happen in the same open space. Ribosomes can latch onto a messenger RNA and start building a protein before the RNA is even finished being copied from the DNA. These two processes run simultaneously, like reading a scroll while someone is still writing it.
Eukaryotic cells can’t do this because the nucleus creates a physical barrier. Transcription happens inside the nucleus, where the RNA is also edited and processed. Only after this processing is complete does the finished messenger RNA get exported through the nuclear membrane to the cytoplasm, where ribosomes translate it into protein. This spatial separation gives eukaryotic cells an extra layer of quality control over gene expression.
Ribosomes
Both cell types use ribosomes to build proteins, but the ribosomes differ in size. Prokaryotic ribosomes are smaller, classified as 70S (a measure of how fast they settle in a centrifuge). They’re made of a small 30S subunit and a large 50S subunit. Eukaryotic ribosomes are larger at 80S, with 40S and 60S subunits, and have roughly twice the molecular mass. This size difference is medically important: many antibiotics work by targeting the smaller 70S ribosomes in bacteria without affecting the 80S ribosomes in human cells.
Cell Walls and External Structures
Most prokaryotes have a rigid cell wall containing peptidoglycan, a mesh-like molecule made of sugar chains cross-linked by short protein fragments. Peptidoglycan is found exclusively in bacterial cell walls and nowhere else in nature. Some eukaryotes also have cell walls, but the chemistry is completely different. Plant cell walls are made of cellulose, and fungal cell walls are made of chitin. Animal cells have no cell wall at all.
The Cytoskeleton
Eukaryotic cells have an elaborate internal scaffolding made of protein filaments: microtubules, microfilaments, and intermediate filaments. This cytoskeleton gives the cell its shape, enables movement, and provides the mechanical infrastructure for pulling chromosomes apart during division. Prokaryotes were long thought to lack a cytoskeleton entirely, but researchers have since identified simpler versions. Bacterial proteins called FtsZ and MreB are structurally related to the tubulin and actin found in eukaryotic cells, and they help with cell division and shape maintenance. Current thinking is that eukaryotic cytoskeletal proteins likely evolved from these prokaryotic ancestors.
An Evolutionary Connection
Despite their differences, prokaryotes and eukaryotes are connected by a remarkable evolutionary event. The endosymbiotic theory proposes that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by an ancestral cell. Over time, these captured cells became permanent residents, eventually evolving into the organelles we see today. The strongest evidence for this comes from the fact that both mitochondria and chloroplasts have their own DNA, reproduce by dividing independently, and have double membranes consistent with being swallowed by another cell. The protein import machinery that moves molecules into these organelles points to a single origin for each, meaning this partnership likely happened once for mitochondria and once for chloroplasts, then spread to all descendant lineages.
In practical terms, every plant and animal cell on Earth carries the descendants of ancient bacteria inside it, still generating energy billions of years after the original partnership began.