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Chapter 26 The Tree of Life: - page 12 / 18





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Concept 26.4 Eukaryotic cells arose from symbioses and genetic exchanges between prokaryotes

Eukaryotic cells differ in many respects from the smaller cells of bacteria and archaea.

Even the simplest single-celled eukaryote is far more complex in structure than any prokaryote.

While there is some evidence of earlier eukaryotic fossils, the first clearly identified eukaryote appeared about 2.1 billion years ago.

Other fossils that resemble simple, single-celled algae are slightly older (2.2 billion years) but may not be eukaryotic.

Traces of molecules similar to cholesterol are found in rocks dating back 2.7 billion years.

Such molecules are found only by aerobically respiring eukaryotic cells.

If confirmed, this would place the earliest eukaryotes at the same time as the oxygen revolution that changed the Earth’s environment so dramatically.

Prokaryotes lack internal structures such as the nuclear envelope, endoplasmic reticulum, and Golgi apparatus.

They have no cytoskeleton and are unable to change cell shape.

Eukaryotic cells have a cytoskeleton and can change shape, enabling them to surround and engulf other cells.

The first eukaryotes may have been predators of other cells.

A cytoskeleton enables a eukaryotic cell to move structures within the cell and facilitates the movement of chromosomes in meiosis and mitosis.

Mitosis made it possible to reproduce the large eukaryotic genome.

Meiosis allowed sexual recombination of genes.

How did the complex organization of the eukaryotic cell evolve from the simpler prokaryotic condition?

A process called endosymbiosis probably led to mitochondria and plastids (the general term for chloroplasts and related organelles).

The endosymbiotic theory suggests that mitochondria and plastids were formerly small prokaryotes living within larger cells.

The term endosymbiont is used for a cell that lives within a host cell.

The proposed ancestors of mitochondria were aerobic heterotrophic prokaryotes.

Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-12

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