Prokaryotic cells are defined by the absence of a membrane-bound nucleus. Their DNA floats freely in the cytoplasm rather than being enclosed in a nuclear envelope, and they lack the complex internal compartments found in plant and animal cells. Despite this simpler architecture, prokaryotes are extraordinarily successful organisms that include all bacteria and archaea. Here are the characteristics that set them apart.
Small Size
Prokaryotic cells range from 0.1 to 5.0 micrometers in diameter, making them significantly smaller than eukaryotic cells, which typically span 10 to 100 micrometers. This compact size gives prokaryotes a high surface-area-to-volume ratio, which allows nutrients and waste to move in and out of the cell efficiently without the need for elaborate internal transport systems.
No Membrane-Bound Nucleus
The most fundamental characteristic of a prokaryotic cell is the absence of a true nucleus. Instead of housing DNA inside a double-membrane envelope, prokaryotes concentrate their genetic material in a region called the nucleoid. The nucleoid has no defined borders. It’s an amorphous mass of DNA suspended in the cytoplasm, visible under a microscope but not separated from the rest of the cell by any membrane.
Circular DNA and Compact Organization
A typical prokaryotic chromosome is a single, circular molecule of DNA. This contrasts sharply with eukaryotic cells, which carry multiple linear chromosomes. Prokaryotic cells are also haploid, meaning they have just one copy of their chromosome rather than paired sets.
Eukaryotic cells package their DNA tightly around proteins called histones, coiling it into structured fibers. Prokaryotes use a different strategy. They rely on small proteins that bend and compress DNA at sharp angles, keeping the chromosome maximally compacted at all times. Even while being actively read and copied, the prokaryotic chromosome stays densely packed in its oval-shaped nucleoid.
Plasmids: Extra Rings of DNA
Beyond the main chromosome, many prokaryotes carry plasmids, which are small, circular DNA molecules that replicate independently. Plasmids don’t contain genes needed for basic growth. Instead, they carry bonus instructions that help cells survive specific challenges, like exposure to antibiotics or toxic heavy metals such as mercury and cadmium.
Plasmids vary enormously in size, from tiny rings carrying just two or three genes to large molecules holding 400 genes or more. Their real significance lies in mobility. Prokaryotic cells can transfer plasmids to one another, even across unrelated species. This is a major reason antibiotic resistance spreads so quickly through bacterial populations. A single resistance plasmid can move productively to most, if not all, types of a bacterial group, assembling arrays of resistance genes along the way.
No Membrane-Bound Organelles
Prokaryotic cells lack the organelles that define eukaryotic cells: no mitochondria, no endoplasmic reticulum, no Golgi apparatus. All chemical reactions happen directly in the cytoplasm or at the cell membrane. Some prokaryotes do form inclusions, which are simple internal compartments that help organize the cytoplasm, but these are far less complex than true organelles.
Smaller Ribosomes
Both prokaryotic and eukaryotic cells build proteins using ribosomes, but their ribosomes differ in size. Prokaryotic ribosomes are classified as 70S, made from a large 50S subunit and a small 30S subunit. Eukaryotic ribosomes are larger, at 80S. This size difference is medically important because many antibiotics specifically target the smaller 70S ribosomes, killing bacteria without harming human cells.
The Cell Wall
Most prokaryotes are surrounded by a rigid cell wall that sits outside the cell membrane and provides structural support. In bacteria, this wall is built from peptidoglycan, a mesh-like polymer made of alternating sugar molecules linked by short chains of amino acids. The thickness of this peptidoglycan layer is the basis for a classic laboratory test: Gram staining. Gram-positive bacteria have a thick peptidoglycan wall that retains the stain’s purple dye. Gram-negative bacteria have a thinner peptidoglycan layer sandwiched between two membranes, and they stain pink instead.
Archaea, the other major group of prokaryotes, typically lack peptidoglycan entirely. Some build their walls from a similar compound called pseudomurein, while others rely on a protein coat called an S-layer. This difference in wall chemistry is one of the clearest ways to distinguish the two prokaryotic domains from each other.
Unique Membrane Lipids in Archaea
Bacterial and archaeal cell membranes also differ at the chemical level. Bacterial membranes are built from fatty acids linked to a glycerol backbone, similar to eukaryotic membranes. Archaeal membranes use a completely different architecture: branched chains linked to glycerol through a different type of chemical bond. This gives archaeal membranes exceptional stability, which helps explain why many archaea thrive in extreme environments like hot springs and salt flats.
Flagella, Pili, and Surface Structures
Many prokaryotes have external appendages that extend from the cell surface. Flagella are long, whip-like structures built from proteins called flagellins. They rotate like propellers to move the cell through liquid environments. Archaea have a functionally similar but structurally distinct version called the archaellum.
Pili (also called fimbriae) are shorter, thinner, hair-like projections found primarily on Gram-negative bacteria. Common pili are short and numerous, and they help cells stick to surfaces, including the lining of your throat, gut, or urinary tract. This adhesion is a critical first step in bacterial infection. Sex pili are much longer but far fewer in number, typically one to six per cell. They physically connect two bacterial cells during conjugation, the process by which plasmids and other DNA are transferred from one cell to another.
Binary Fission Instead of Mitosis
Prokaryotic cells reproduce by binary fission, a simpler process than the mitosis used by eukaryotic cells. The concept is straightforward: a cell grows to roughly twice its starting size, copies its DNA, moves the two copies to opposite ends, and splits down the middle. A ring of protein assembles at the cell’s center and pinches inward, dividing the cytoplasm while new cell wall material is synthesized at the division site.
Despite its simplicity, binary fission is tightly regulated. The timing of DNA replication, the positioning of the division ring, and the synthesis of new wall material are all coordinated to ensure each daughter cell gets a complete chromosome without damage. Under ideal conditions, some bacteria complete this entire cycle in as little as 20 minutes, allowing populations to grow exponentially.
A Prokaryotic Cytoskeleton
For decades, scientists assumed prokaryotes lacked any internal skeleton. That turned out to be wrong. Prokaryotic cells contain structural proteins that are distant relatives of the cytoskeletal proteins in eukaryotic cells. One protein forms the division ring that pinches cells apart during binary fission. Another guides where new cell wall material gets deposited, directly determining whether a bacterium is rod-shaped, curved, or spherical. When the gene for this shape-determining protein is disabled in rod-shaped bacteria, the cells become spheres. These proteins demonstrate that even the simplest cells require internal organization to maintain their structure and divide reliably.