Prokaryotic cell structure

Prokaryotic cells are much smaller than eukaryotic cells. They have no nucleus, and their cytoplasm contains no membrane-bound organelles. Their cytoplasm does contain ribosomes, which make proteins and are smaller than those in eukaryotic cells.

DNA in prokaryotic cells

A prokaryotic cell has a single main circular DNA molecule. Because there is no nucleus, this DNA lies free in the cytoplasm and is not associated with proteins.

Many prokaryotic cells also contain one or more plasmids. These are small circular DNA molecules, separate from the main DNA molecule.

Prokaryotic cell wall and external structures

The cell wall contains murein, also called peptidoglycan. This is a polymer made of sugars linked by short peptide chains.

Some prokaryotic cells have a capsule surrounding the cell, one or more flagella, or both. Flagella can help the cell move.

Virus particle structure

A virus particle is not a cell: it is acellular. Viruses are considered non-living because they cannot carry out life processes or reproduce independently; they depend on host cells to make new virus particles.

Each virus particle contains genetic material, which may be DNA or RNA. This genetic material is enclosed in a protein coat called a capsid. The capsid protects the genetic material.

Attachment proteins on the virus particle bind to matching structures on the surface of a host cell. This binding allows the virus to attach to the cell.