3.2.1.2 Structure of prokaryotic cells and of viruses

Prokaryotic cells
Prokaryotic cells are much smaller than eukaryotic cells. They also differ from eukaryotic cells as they:

• lack membrane-bound organelles
• have smaller (70S) ribosomes
• have no nucleus (instead they have a single circular DNA molecule that is free in the cytoplasm and not associated with proteins)
• a cell wall containing murein (a glycoprotein)

Some prokaryotic cells might additionally have:

• one or more plasmids
• a capsule surrounding the cell
• one or more flagella

Viruses
These are acellular non living particles. They are smaller than bacteria (20-300nm) and contain nucleic acids (DNA or RNA) as genetic material. They can only multiply inside a host cell. The nucleic acid is enclosed in a protein coat (a capsid). Some viruses are further surrounded by a lipid envelope (e.g HIV). The lipid envelope/capsid has attachment proteins which allow the virus to identify and attach to a host cell.