AQA A-Level Biology: Prokaryotic Cells and Viruses
A clear revision guide to prokaryotic cells and viruses for AQA A-Level Biology: prokaryote structure, how prokaryotes compare with eukaryotes, and why viruses are acellular.
Not every cell keeps its DNA in a nucleus. Prokaryotic cells, which include all bacteria and archaea, are much smaller and simpler than eukaryotic cells, and they are always single-celled. Viruses are simpler still, and are not really cells at all. This guide covers both, and how they differ from the eukaryotic cells in the cell structure guide.
The structure of a prokaryotic cell
The defining feature of a prokaryotic cell is what it lacks: it has no nucleus and no membrane-bound organelles. Its genetic material sits free in the cytoplasm.
Every prokaryotic cell has:
- a cell-surface membrane
- a cell wall made of murein (a glycoprotein), which supports the cell
- cytoplasm
- small 70S ribosomes
- a single loop of circular DNA, free in the cytoplasm and not wound around proteins
Some prokaryotes also have extra structures that are not always present:
- a capsule surrounding the cell wall, which helps protect the cell
- one or more plasmids, small separate rings of DNA that can be exchanged between cells
- a flagellum, a tail-like structure used for movement
A generalised prokaryotic cell showing the cell wall, cell-surface membrane, cytoplasm, circular DNA, ribosomes, and the sometimes-present capsule, plasmids and flagellum
Prokaryotic and eukaryotic cells compared
The two cell types share the basics of life, a membrane, cytoplasm, ribosomes and DNA, but differ in almost every detail of how those are arranged.
| Feature | Eukaryotic cell | Prokaryotic cell |
|---|---|---|
| Membrane-bound organelles | Present, for example mitochondria and ER | Absent |
| Nucleus | Present, contains the DNA | Absent, DNA is free in the cytoplasm |
| DNA | Long, linear, wound around histone proteins | Short, circular, not associated with proteins |
| Ribosomes | Larger, 80S | Smaller, 70S |
| Cell wall | Only in plants, algae and fungi (cellulose or chitin) | In all prokaryotes, made of murein |
| Plasmids and capsule | Not present | Sometimes present |
| Overall size | Larger | Much smaller |
Why viruses are described as acellular
Viruses sit outside the living world as biologists usually define it. They are acellular, meaning they are not made of cells: they have no cell membrane, no cytoplasm and no organelles. They are also considered non-living because they have no metabolism of their own and cannot move, respire, excrete or replicate independently. A virus can only make copies of itself by taking over a host cell.
The structure of a virus
A virus particle is remarkably economical. At its core is a strand of nucleic acid, which may be DNA or RNA, carrying the instructions to make more virus. This is enclosed in a protective protein coat called a capsid.
On the outside are attachment proteins, which let the virus bind to specific receptor molecules on the surface of a host cell. Because binding depends on a match between the attachment protein and the receptor, most viruses can only infect particular host cells. Some viruses, such as HIV, are also surrounded by a lipid envelope taken from a previous host cell.
A generalised virus particle showing the nucleic acid core, the surrounding capsid and the attachment proteins, with a lipid envelope shown as an option
Because a virus has no cell structures and no metabolism of its own, it can only reproduce inside a host. How it does that, and how prokaryotes divide, is covered in the cell cycle and mitosis guide.