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asexually and sexually.

in diameter); and a bacterial cell (cylinder shaped, 2 pm long, 0.8 pm in diameter).

Science in Action

How Did Eukaryotic Cells Evolve?

The idea that cellular organelles were originally separate organisms was first proposed in the early 1900s. The few scientists who later became aware of this hypothesis thought that it was preposterous. In the 1960s, Lynn Margulis thought that the idea should be reexamined. Her research led her to form a theory that shook the foundations of the biological community.

Lynn Margulis

HYPOTHESIS: Cellular Organelles Were the Result of Endosymbiosis

Endosymbiosis is a relationship in which one organism lives inside another. Lynn Margulis, an American biologist, thought that endosymbiosis led to the evolution of eukaryotic cells. Margulis proposed that mitochondria and chloroplasts, the cell's energy-producing organelles, were initially separate organisms that had become integral parts of larger cells.

Margulis hypothesized that the first cells with chloroplasts probably came about when a host organism ingested photosynthetic bacteria. A few of these bacteria were not digested. These unlikely partnerships provided something for everyone: The guest gave the host a source of food, while the host protected the guest. Margulis also surmised that mitochondria arose in a similar way.

METHODS: Use Electron Microscopy to Observe Organelles and Cells

Margulis took advantage of advances in electron microscopy and molecular biology to obtain evidence in support of her hypothesis. Working with her graduate research advisor, Hans Ris, Margulis used high-powered electron microscopy to carefully observe and compare mitochondria, chloroplasts, eukaryotic cells, and prokaryotic cells. In addition, she looked for the presence of DNA in the chloro-plasts of Euglena gracilis by using radioactively labeled nucleotides.

RESULTS: Organelles Are Similar to Bacteria

Margulis found that bacteria and organelles show many similarities. Like prokaryotes, mitochondria and chloroplasts have circular DNA. These organelles also have their own ribosomes, which are the same size as those found in prokaryotes but smaller than those found in the cytoplasm of eukary-otes. Both mitochondria and chloroplasts are found enclosed in membranes as though they were captured in a vacuole of a larger cell. Also, mitochondria and chloroplasts divide by fission, as bacteria do.

An endosymbiotic relationship was witnessed by Kwang Jeon at the University of Tennessee.

One day, Jeon noticed that bacteria had infected his amoe-bas and that a few of the amoebas did not die. Their new partners replicated along with the hosts. Generations later, these amoebas are thriving; yet the formerly separate organisms are totally dependent upon one another. If the invader is removed from the host, both die.

CONCLUSION: Organelles Arose Through Endosymbiosis

Today, endosymbiosis is the accepted explanation for how eukaryotes arose. Additional support for Margulis's theory comes from examining modern organisms that contain intracellular symbiotic bacteria and photosynthetic protists. For example, sea slugs incorporate chloroplasts into some of their cells, which provides a source of energy for the sea slugs when other food is scarce.

Endosymbiosis led to the formation of eukaryotic cells. Multicellular eukaryotes, such as these fossil algae, arose around 2 billion years ago.

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