Lessons 15–16 · 2 lessons · A. Zikiryayev, A. To‘xtayev, I. Azimov, N. Sonin. Biology. Basics of Cytology and Genetics, Grade 9, revised and expanded 5th edition. “Yangiyul Poligraph Service”, Tashkent, 2019
16
Evolution of cells
Textbook: pp. 49–50
GoalExplain the hypotheses about the origin of eukaryotic cells and give the evidence.
According to palaeontological data, the oldest prokaryotic cells appeared about 3.5 billion years ago; eukaryotes are thought to have arisen later, about 1.5–2 billion years ago. By the symbiosis (endosymbiosis) hypothesis, mitochondria came from aerobic prokaryotes and chloroplasts from photosynthesising prokaryotes: they entered a larger host cell and moved to a mutually beneficial life together. Evidence: they have their own circular DNA, RNA and ribosomes, they multiply by division, and they have double membranes. By the invagination hypothesis, the nuclear envelope, endoplasmic reticulum and other membrane structures formed from the outer membrane folding inward; in the multi-genome hypothesis the eukaryotic genome is explained by a prokaryotic genome splitting into parts. In current science the symbiosis hypothesis is widely accepted for mitochondria and plastids, while the origin of the nucleus and other structures is still being studied. Since about 2015, findings on the closeness of archaea (the Asgard group) to eukaryotes have added new evidence to this research.
Worked examples
The alga Chlorella lives in the cytoplasm of some ciliates and feeds the host by photosynthesis: a present-day example of symbiosis inside a cell.
Mitochondrial ribosomes resemble bacterial ribosomes, and some antibiotics can affect their work too; this supports their origin from prokaryotes.
Class activity
“Hypothesis court”. Three groups defend three hypotheses (symbiosis, invagination, multi-genome) while the rest ask questions. At the end the class decides which hypothesis is backed by good evidence for which organelle.
Practice
1
The origin of which organelles is explained by the symbiosis hypothesis?
Mitochondria and chloroplasts.
2
Name two pieces of evidence supporting the symbiosis hypothesis.
They have their own circular DNA, RNA and ribosomes; they divide like bacteria; they have two membranes.
3
Which eukaryotic structures does the invagination hypothesis explain easily?
Internal membranes such as the nuclear envelope and the endoplasmic reticulum. It also tries to explain the double membranes of mitochondria and chloroplasts this way, but for these the symbiosis hypothesis has stronger evidence.
4
Why is the word “hypothesis” used rather than “proven fact”?
Cells arose billions of years ago and this cannot be observed directly, so these are explanations based on indirect evidence that are tested against new data.