Prokaryotes are unicellular organisms whose genetic material is not enclosed by a membrane. Instead of a true nucleus, their DNA resides in a nucleoid, and they lack membrane‑bound organelles.
Structural and genetic characteristics
Cells classified as prokaryotes comprise the domains Bacteria and Archaea. They are typically one to five micrometres in size and possess a rigid cell wall composed of peptidoglycan in bacteria or pseudomurein and other polymers in archaea. The absence of a membrane‑bound nucleus means their chromosome—a single circular DNA molecule—lies free in the cytoplasm within a dense region called the nucleoid. Many prokaryotes also harbour plasmids that provide accessory functions such as antibiotic resistance. Their ribosomes are 70S, smaller than the 80S ribosomes of eukaryotic cells. Energy production occurs at the plasma membrane and in the cytoplasm, rather than in mitochondria or chloroplasts. Prokaryotes reproduce asexually by binary fission, allowing rapid population growth and genetic changes through mutation, horizontal gene transfer and recombination. Their metabolic diversity is vast: they can be aerobic or anaerobic, phototrophic or chemotrophic, and can use a wide variety of electron donors and acceptors.
Diversity and ecological roles
Bacteria and archaea inhabit virtually every environment, from soil and freshwater to hydrothermal vents and saline lakes. Morphologically they may be cocci, bacilli, spirilla, vibrios or filamentous. Cyanobacteria perform oxygenic photosynthesis and were responsible for the ancient oxygenation of Earth’s atmosphere. Nitrogen‑fixing bacteria such as Rhizobium form symbioses with legumes, converting atmospheric nitrogen into bioavailable forms. Extremophilic archaea thrive at high temperatures, high salinity or low pH, illustrating adaptations beyond the capabilities of eukaryotes. Model organisms like Escherichia coli have been pivotal in understanding gene regulation and molecular biology. In medicine, prokaryotes include commensal microbiota essential for health and pathogens causing diseases such as tuberculosis and cholera. Their enzymes and metabolic pathways are exploited in biotechnology for fermentation, bioremediation and production of pharmaceuticals.
Although prokaryotes lack the compartmentalisation seen in eukaryotic cells, their streamlined organisation and metabolic versatility enable them to dominate the biosphere. They represent the ancestral forms of life and provide insights into evolution, ecology and biotechnology.
Related Terms: Eukaryote, Bacteria, Archaea, Nucleoid, Binary Fission