Prokaryotic and eukaryotic cells differ in one fundamental way: eukaryotic cells contain a nucleus that houses their DNA, while prokaryotic cells do not. This single distinction branches into dozens of structural, genetic, and functional differences that separate bacteria from the complex cells making up plants, animals, fungi, and protists. Prokaryotic cells are also dramatically smaller, typically 0.1 to 5.0 micrometers in diameter compared to 10 to 100 micrometers for eukaryotic cells.
Cell Structure and Organization
Both cell types share a few basics: a plasma membrane surrounding the cell, cytoplasm filling the interior, ribosomes for building proteins, and DNA carrying genetic instructions. Beyond that, their architecture diverges sharply.
Prokaryotic cells are relatively simple. They have no internal compartments separated by membranes. Their DNA floats in a region of the cytoplasm called the nucleoid, and they lack a cytoskeleton, the internal scaffolding that gives eukaryotic cells their shape and ability to move materials around internally.
Eukaryotic cells, by contrast, are packed with membrane-enclosed organelles, each performing specialized tasks. The nucleus stores and protects DNA behind a double membrane studded with pores that control what enters and exits. Mitochondria generate most of the cell’s energy. The endoplasmic reticulum folds new proteins (when studded with ribosomes) or synthesizes fats (when smooth). The Golgi apparatus modifies and sorts molecules before shipping them to their final destinations inside or outside the cell. Lysosomes act as recycling centers, breaking down worn-out cell parts and foreign material with digestive enzymes.
How DNA Is Organized
Prokaryotes carry their genome on a single, circular chromosome. It’s a relatively compact arrangement, with the DNA loosely associated with proteins that help keep it organized inside the cell.
Eukaryotes store their DNA across multiple linear chromosomes. In humans, that’s 46 chromosomes per cell. This DNA is tightly wound around small proteins called histones, which spool the long DNA strands into compact, orderly packages. Histones allow eukaryotic cells to fit enormous amounts of genetic information into the nucleus and play a role in controlling which genes get turned on or off. Prokaryotic bacteria lack true histones, though they use other proteins to achieve a similar compacting effect.
Cell Walls and Outer Barriers
Many prokaryotic and eukaryotic cells have cell walls, but the materials are completely different. Bacterial cell walls contain peptidoglycan, a mesh-like polymer made of sugars and amino acids. Some bacteria (called gram-positive) have a thick peptidoglycan layer, while others (gram-negative) have a thinner layer plus an additional outer membrane containing large molecules called lipopolysaccharides. This structural difference is the basis of the gram staining technique used to classify bacteria in labs.
Eukaryotic cell walls, when present, use simpler ingredients. Plant and algal cell walls are built from cellulose. Fungal cell walls are made of chitin, the same tough material found in insect exoskeletons. Animal cells have no cell wall at all, relying solely on their plasma membrane and cytoskeleton for structure.
Ribosomes
Both cell types use ribosomes to translate genetic instructions into proteins, but the ribosomes themselves are different sizes. Prokaryotic ribosomes are classified as 70S, while eukaryotic ribosomes are larger at 80S. (The “S” stands for Svedberg units, a measure of how fast a particle settles in a centrifuge, reflecting its size and shape.) This size difference is medically important: many antibiotics work by targeting the smaller 70S ribosomes in bacteria without interfering with the 80S ribosomes in human cells.
Energy Production
Cells need to produce ATP, the molecule that powers nearly every cellular process. Where that production happens is one of the clearest dividing lines between the two cell types.
In prokaryotes, the molecular machinery for making ATP sits on the cell’s plasma membrane. The electron transport chain and the enzyme that actually assembles ATP molecules are both embedded there. This means the cell membrane does double duty: it acts as both the cell’s outer barrier and its energy-producing surface.
Eukaryotic cells offload energy production to mitochondria, organelles with their own double membrane. The ATP-generating machinery is confined to the inner mitochondrial membrane, which is folded into ridges that dramatically increase its surface area. This arrangement lets eukaryotic cells produce far more energy than a prokaryote of comparable size, supporting their larger size and greater complexity.
How Each Cell Type Divides
Prokaryotes reproduce through binary fission, a straightforward process. The single circular chromosome is copied, the two copies move to opposite ends of the cell, and the cell pinches in half to form two identical daughter cells. There’s no elaborate internal machinery involved. The DNA attaches directly to the cell membrane, and the whole process can happen remarkably fast, sometimes in as little as 20 minutes under ideal conditions.
Eukaryotic cell division is far more elaborate. The process, called mitosis, unfolds in distinct stages: prophase, metaphase, anaphase, and telophase. During prophase, chromosomes condense into visible structures and a spindle apparatus made of protein fibers begins to form. By metaphase, chromosomes line up along the cell’s equator, with a built-in checkpoint ensuring every chromosome is properly attached before proceeding. During anaphase, the spindle fibers pull each chromosome copy to opposite poles of the cell. Finally, a new nuclear envelope forms around each set of chromosomes, and the cytoplasm divides. The entire process includes multiple quality-control checkpoints that help prevent errors in chromosome distribution.
Why the Differences Exist
The evolutionary relationship between these two cell types is more intertwined than you might expect. Endosymbiotic theory, first proposed over a century ago, holds that mitochondria and chloroplasts (the organelles responsible for photosynthesis in plant cells) were once free-living prokaryotes. At some point, a larger cell engulfed these smaller prokaryotes, and instead of digesting them, the two formed a mutually beneficial partnership. Over billions of years, the engulfed prokaryotes became permanent residents, losing their independence but retaining their own DNA and double membranes.
The strongest evidence for this comes from the protein import machinery that mitochondria and chloroplasts use. Both organelles still carry small, circular genomes resembling bacterial DNA, and they replicate independently of the rest of the cell. Gene comparisons consistently show that mitochondria are related to a group of bacteria, while chloroplasts trace back to photosynthetic bacteria called cyanobacteria. In other words, eukaryotic cells aren’t just different from prokaryotic cells. They’re partly built from them.