Prokaryotic cells and viruses
Introduced| English | Meaning |
|---|---|
| prokaryotic/ˌprɒkərɪˈɒtɪk/ | Describing cells that lack a membrane-bound nucleus and other membrane-bound organelles. |
| unicellular/ˌjuːnɪˈseljʊlə/ | unicellular |
| peptidoglycan/ˈpeptɪdɒɡlɪkən/ | A material made of sugar chains cross-linked by short peptides that gives most bacterial cell walls their strength. |
| nucleic acid/njuːˈklɪɪk ˈæsɪd/ | A polymer of nucleotides that stores or transmits genetic information, such as DNA or RNA. |
| capsid/ˈkæpsɪd/ | The protein coat surrounding a virus's genetic material. |
| envelope/ˈenvələʊp/ | A membrane-based covering; in an enveloped virus, a lipid layer surrounds the capsid. |
| host/həʊst/ | An organism in or on which another organism or infectious agent lives or reproduces. |
Prokaryotes and viruses
- Not all cells are eukaryotic. Bacteria are prokaryotic 原核 — smaller and simpler.
- Viruses are not even cells.
- Knowing the differences is a favourite exam comparison.
Prokaryotic cells (bacteria)
- A prokaryotic cell is unicellular 单细胞 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 肽聚糖 (not cellulose).
- No double-membrane organelles — so no nucleus, no mitochondria, no chloroplasts.
A prokaryotic cell: circular DNA lies free, with no nucleus or membrane-bound organelles
Explore a bacterium
Tap each part of this prokaryotic cell — note there is no nucleus and no membrane-bound organelles.
A key feature of a prokaryotic cell is that it:
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Prokaryotes have no nucleus and no double-membrane organelles; their DNA is circular and free.
Prokaryotes have smaller 70S ribosomes, while the cytoplasm of eukaryotes has larger 80S ribosomes.
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Correct — 70S in prokaryotes; 80S in eukaryotic cytoplasm (mitochondria and chloroplasts contain 70S, a clue to their origin).
The small extra ring of DNA in a bacterium, separate from its main loop, is a ______.
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Plasmids carry extra genes (e.g. antibiotic resistance) and can be passed between bacteria.
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 |
Match each feature to the cell type it belongs to.
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The defining difference is the nucleus — absent in prokaryotes, present in eukaryotes, which also have membrane-bound organelles and 80S ribosomes.
Viruses
- All viruses are non-cellular — not made of cells.
- Each has a core of nucleic acid 核酸 (either DNA or RNA, never both) inside a protein coat (capsid 衣壳); some have a lipid envelope 包膜.
- A virus has no cytoplasm, organelles or ribosomes — it cannot respire or make proteins, and can only copy itself inside a living host 宿主 cell.
A generalised virus: a nucleic acid core inside a protein capsid, with a lipid envelope in some viruses
A virus consists of:
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Viruses are non-cellular: nucleic acid + capsid (± lipid envelope), with no organelles.
The protein coat that surrounds a virus's nucleic acid is called the ______.
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The capsid is the protein coat; some viruses also have an outer lipid envelope.
Why can a virus only reproduce inside a host cell?
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With no ribosomes/organelles, a virus must hijack a host cell's machinery to copy itself.
You've got it
- 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