Prokaryotes are the oldest living organisms on Earth, dating back to 3.5 billion years ago. They function as single cells; in other words, they are unicellular. Each cell is only about 1 micrometer (μm) in size!

Prokaryotes are part of the kingdoms Archaea & Eubacteria.

Prokaryotes have this basic structure:

Prokaryotes have:

  • a plasma membrane (or cell membrane), which controls the movements of molecules in and out of the cell. (read about Membranes)

  • a cell wall, which provides the cell with additional support and shape. Some bacteria have a further mucous-like outer layer made of polysaccharides, called an outer membrane or capsule, that protects the cell from phagocytosis.

  • projections such as flagella to help them move, and pili for adhering to surfaces and genetic exchange (“sex”).

Inside a prokaryotic cell, there are:

  • free DNA. The DNA of prokaryotes is circular, like a rubber band. The region that contains the DNA is called the nucleoid (there is no nucleus!).

  • plasmids, which are small DNA circles that contain genes that give the bacteria an advantage (e.g. antibiotic resistance). Plasmids are “read” separately from the chromosomal DNA and are used in genetic engineering.

  • free ribosomes that are involved in making proteins (protein synthesis).

Prokaryotes do NOT have any membrane-bound organelles, such as mitochondria or endoplasmic reticulum.

Prokaryotes may have other components in their cytoplasm that allow them to carry out special metabolic processes such as photosynthesis, nitrogen fixation and fermentation.

E. Coli bacteria

This electron microscope image shows rod-shaped Escherichia Coli (E. Coli) bacteria that can be found in our intestines. Some strains are harmless and help us make Vitamin K, but some strains are pathogenic and can cause food poisoning. Each cell is about 2 micrometers (?m) in length. In this image, you can see some cells that are replicating by binary fission, where the cell first replicates its DNA, and then divides into 2 separate cells.

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