3.5—Prokaryotic cell ultrastructure

Prokaryotic cells are smaller cells without a nucleus or membrane-bound organelles. Their circular bacterial DNA lies in the cytoplasm, their ribosomes are 70S, and their cell wall contains murein (peptidoglycan).

Some prokaryotes also have plasmids, a capsule, flagella or pili. Plasmids are small DNA loops; a capsule can protect the cell; flagella support movement; pili can help attachment or DNA transfer. These structures are not present in every bacterium.

A plasmid can carry a resistance gene independently of the main circular chromosome. That explains why a bacterium may acquire a useful trait without changing the definition of its chromosome.

“Prokaryote” does not mean “no DNA” or “no membrane”: it has a cell-surface membrane and genetic material, but no membrane-bound nucleus. Size alone is not a structural comparison.