Prokaryotic and Eukaryotic Cells

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Prokaryotic cells, bacteria and archaea, are small and simple, with no nucleus: their DNA floats in a region called the nucleoid, and they lack membrane-bound organelles. Eukaryotic cells, found in animals, plants, fungi and protists, are larger and have a nucleus and organelles such as mitochondria.

Both have a cell membrane, cytoplasm, ribosomes and DNA. Prokaryotes reproduce by binary fission. The endosymbiotic theory suggests mitochondria and chloroplasts were once free-living prokaryotes engulfed by early eukaryotes.

A typical bacterium such as E. coli is about 2 micrometres long, while a human cheek cell is roughly ten times wider. That size difference exists partly because eukaryotes use internal compartments to run many reactions at once, like rooms in a house, whereas prokaryotes do everything in one open space. Bacterial cells also carry small rings of DNA called plasmids that can be passed between cells.

Understanding the difference matters for medicine: antibiotics such as penicillin target bacterial cell walls or ribosomes, which human cells lack or have in different forms, so they harm bacteria but not us. A frequent mistake is saying prokaryotes have no DNA; they do, it simply is not enclosed in a nucleus. Another is thinking all bacteria are harmful, when many live in our gut and help digest food.