Prokaryotes and Eukaryotes
Two Domains of Cellular Life
Prokaryotic Cells
Prokaryotes (Bacteria and Archaea) are typically 1–10 µm in size. Key features:
- No membrane-bound nucleus — DNA is in the nucleoid region
- No membrane-bound organelles
- Circular DNA (chromosome) plus often small plasmid rings
- Cell wall (peptidoglycan in bacteria)
- Ribosomes (70S type: 30S + 50S subunits)
- Reproduce by binary fission
Despite apparent simplicity, prokaryotes are metabolically diverse and represent the most abundant life on Earth.
Eukaryotic Cells
Eukaryotes (plants, animals, fungi, protists) are 10–100 µm in size. Defining features:
- Membrane-bound nucleus containing linear chromosomes
- Membrane-bound organelles: mitochondria, ER, Golgi, lysosomes (animals), chloroplasts (plants)
- 80S ribosomes (40S + 60S)
- Cytoskeleton (microfilaments, intermediate filaments, microtubules)
- Reproduce by mitosis/meiosis
Eukaryotes arose ~1.5–2 billion years ago, likely from an ancient prokaryote engulfing another (endosymbiotic theory).
Endosymbiotic Theory
Mitochondria and chloroplasts are thought to have originated as free-living prokaryotes engulfed by ancestral eukaryotic cells. Evidence: they have their own circular DNA, 70S ribosomes, and reproduce by binary fission — all prokaryotic features.
Prokaryote vs Eukaryote Comparison
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | No (nucleoid only) | Yes (membrane-bound) |
| Size | 1–10 µm | 10–100 µm |
| Organelles | No membrane-bound organelles | Many (mitochondria, ER, Golgi etc.) |
| Ribosomes | 70S | 80S |
| DNA | Circular, no histones | Linear, with histones |
| Cell wall | Usually (peptidoglycan) | Varies (cellulose in plants; none in animals) |
| Reproduction | Binary fission | Mitosis / Meiosis |
Which feature is EXCLUSIVE to eukaryotic cells?
Only eukaryotes have a membrane-bound nucleus. Both prokaryotes and eukaryotes have ribosomes and a cell membrane.
Correct answer: Membrane-bound nucleus
The endosymbiotic theory proposes that mitochondria originated from:
Mitochondria share structural and genetic similarities with alpha-proteobacteria, supporting their origin as endosymbionts.
Correct answer: Ancient engulfed prokaryotes
Which ribosome type is found in prokaryotes?
Prokaryotes have 70S ribosomes (30S+50S subunits). Eukaryotes have 80S ribosomes (40S+60S) in the cytoplasm.
Correct answer: 70S
Give TWO pieces of evidence supporting the endosymbiotic origin of mitochondria.
These features parallel characteristics of free-living prokaryotes, strongly supporting the endosymbiotic hypothesis.
Correct answer: Any two: own circular DNA; 70S ribosomes; double membrane; divide by binary fission; similar size to bacteria; similar gene sequences to alpha-proteobacteria