Origins of cells: evidence and limits
הוצג| English | משמעות |
|---|---|
| endosymbiosis/ˌendəʊˌsɪmbaɪˈəʊsɪs/ | אנדוסמביוזה |
| inference/ˈɪnfərəns/ | הסקה |
What would explain this observation?
- Mitochondria have their own DNA and divide within cells. These features invite an evolutionary explanation that can be tested against other evidence.
Build the model
- The endosymbiotic model explains mitochondria and chloroplasts as descendants of bacteria living within another cell lineage.
- Double membranes, ribosomes, DNA and division provide several lines of evidence.
- This model is distinct from a complete explanation for the origin of the first life.
- endosymbiosis 内共生: One organism living within another in an evolutionary association.
- inference 推论: An explanation drawn from observations and reasoning.
Which observation is relevant evidence for mitochondrial endosymbiosis?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Mitochondria carrying their own DNA - bacterial-style, with their own ribosomes - is exactly what an engulfed-bacterium origin predicts. Equal cell sizes or a nucleus in every organelle are not observations and would not test the model.
Match each technical term to its precise meaning.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Independent observations strengthen a model when they make a consistent prediction.
- Similarity is evidence to evaluate alongside ancestry, sequence comparisons and alternative explanations, rather than proof from one feature.
In the lab: Use attributed micrographs and sequence data. Separate observations from inferences in a table. Check whether each comparison concerns a mitochondrion, chloroplast, bacterial cell or eukaryotic nucleus.
Which two habits make the investigation or model in this case more defensible?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: 18 of 24 compared sequence sites are identical.
- Identity = 18/24 × 100 = 75%.
- Identity across this short alignment cannot alone date an evolutionary split or establish that one organism is the direct ancestor of another.
Your turn: 21 of 28 compared sites match. Calculate percentage identity.
21 of 28 compared sites match. Calculate percentage identity.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
The result is 75 %. Known: 21 of 28 sites match. Use percentage identity = matches / sites x 100. Identity = 21/28 x 100 = 75%. A short-alignment percentage cannot by itself date a split or prove direct ancestry.
Check the conclusion and its limits
- The endosymbiotic model does not say that every organelle arose from an engulfed bacterium.
- A laboratory model of early chemistry is not a recreation of the whole historical event.
Not true: One matching sequence proves the complete origin of life. One sequence match is one piece of evidence about one locus. Origins models rest on many independent lines - DNA, ribosomes, membranes, biochemistry - not a single comparison.
- The endosymbiotic model explains mitochondria and chloroplasts as descendants of bacteria living within another cell lineage.
- Independent observations strengthen a model when they make a consistent prediction.
- Watch for: the endosymbiotic model does not say that every organelle arose from an engulfed bacterium.
Write from an identity of 80 percent
- 16 of 20 compared sequence sites are identical.
- Find the percentage identity. Say what that percentage does not prove.
One matching sequence proves the complete origin of life.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
One sequence match is one piece of evidence about one locus. Origins models rest on many independent lines - DNA, ribosomes, membranes, biochemistry - not a single comparison.
One organism living within another in an evolutionary association: write the technical term.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
endosymbiosis means One organism living within another in an evolutionary association.
16 of 20 compared sequence sites are identical. What is the percentage identity, and what can that short alignment not prove by itself?
16 out of 20 is 80 percent. Similarity is evidence to weigh, not a finished family tree.
What is the percentage identity?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
16 divided by 20 is 0.80, which is 80 percent.