Prokaryotic cells and viruses
Prokaryotes and viruses
- Not all cells are eukaryotic. Bacteria are prokaryotic — smaller and simpler.
- Viruses are not even cells.
- Knowing the differences is a favourite exam comparison.
Prokaryotic cells (bacteria)
- A prokaryotic cell is unicellular and small (about $1$–$5\ \mu\text{m}$).
- It has no nucleus — its circular DNA lies free in the cytoplasm.
- Smaller (70S) ribosomes, and a cell wall of peptidoglycan (not cellulose).
- No double-membrane organelles — so no nucleus, no mitochondria, no chloroplasts.
Practice
A key feature of a prokaryotic cell is that it:
Prokaryotes have no nucleus and no double-membrane organelles; their DNA is circular and free.
Practice
Prokaryotic cells have which size of ribosome?
Prokaryotes have 70S ribosomes; eukaryotic cytoplasm has 80S (with 70S inside mitochondria/chloroplasts).
Prokaryote vs eukaryote
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| size | ~1–5 µm | ~10–100 µm |
| DNA | circular, free | linear, in a nucleus |
| nucleus | none | present |
| double-membrane organelles | none | mitochondria (+ chloroplasts) |
| ribosomes | 70S | 80S |
Practice
Which difference between prokaryotes and eukaryotes is correct?
The defining difference is the nucleus: present in eukaryotes, absent in prokaryotes.
Viruses
- All viruses are non-cellular — not made of cells.
- Each has a core of nucleic acid (either DNA or RNA, never both) inside a protein coat (capsid); some have a lipid envelope.
- A virus has no cytoplasm, organelles or ribosomes — it cannot respire or make proteins, and can only copy itself inside a living host cell.
Practice
A virus consists of:
Viruses are non-cellular: nucleic acid + capsid (± lipid envelope), with no organelles.
Practice
Why can a virus only reproduce inside a host cell?
With no ribosomes/organelles, a virus must hijack a host cell's machinery to copy itself.
You've got it
Key idea
- prokaryotes = small, no nucleus, circular free DNA, 70S ribosomes, peptidoglycan wall, no double-membrane organelles
- eukaryotes = larger, nucleus, linear DNA, 80S ribosomes
- viruses are non-cellular: nucleic acid (DNA or RNA) + capsid (± envelope)
- a virus needs a host cell to replicate