Bacteria, Prokaryotic Cells and Viruses

A typical bacterial cell has a cell-surface membrane, cytoplasm, small ribosomes and a cell wall containing murein, also called peptidoglycan. Its main DNA molecule is typically circular and lies in a nucleoid region without a nuclear envelope. It lacks the membrane-bound nucleus, mitochondria and rough ER of a eukaryotic cell.

Many bacteria also possess plasmids, small additional DNA molecules that can carry useful genes, a capsule, or flagella. These are not present in every species. Bacteria are often smaller than eukaryotic cells, but size alone is insufficient for identification; use several structural features. The syllabus model describes a typical bacterial prokaryote rather than every exception across all microorganisms.

A virus particle contains genetic material, either DNA or RNA depending on the virus, enclosed in a protein capsid. Attachment proteins bind to suitable receptors on a host cell. Some viruses also have a lipid envelope, so an envelope should not be added automatically to every viral diagram.

Viruses have no independent cellular metabolism or ribosomes and depend on host-cell machinery to produce components and assemble new particles. They do not grow and divide by mitosis or binary fission. In this specification they are described as acellular and non-living, while their capacity for replication and evolution still makes them biologically important.

A bacterial cell can synthesise proteins using its own ribosomes and divide by binary fission under suitable conditions. A virus needs a compatible host cell, and receptor binding helps determine which cells it can infect. A drawing with ribosomes and cytoplasm therefore suggests a cell rather than an isolated virus particle.

Antibiotics target bacterial structures or processes, such as cell-wall formation or particular ribosomal functions. Viruses lack these targets, so antibiotics do not treat a viral infection itself. This does not imply that every bacterium is susceptible to every antibiotic, or that all treatments against viruses are impossible. Keep the mechanism and the scope of a claim separate.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.