UNIT 2: ORIGIN OF EUKARYOTIC CELLS
Prokaryotic cells are simpler and more primitive in their organization than eukaryotic cells. According to the fossil record, prokaryotic organisms antedate, by at least 2 billion years, the first eukaryotes that appeared some 1.5 billion years ago. It seems highly likely that eukaryotes evolved from prokaryotes, and the most likely explanation of this process is the endosymbiotic theory. The basis of this hypothesis is that some eukaryotic organelles originated as free-living prokaryotes that were engulfed by larger cells in which they established a mutually beneficial relationship. For example, mitochondria would have originated as free-living aerobic bacteria and chloroplasts as cyanobacteria, photosynthetic prokaryotes formerly known as blue-green algae.
The endosymbiotic theory provides an attractive explanation for the fact that both mitochondria and chloroplasts contain DNA and ribosomes of the prokaryotic type (Table 1.1). The case for the origin of other eukaryotic organelles is less persuasive. While it is clearly not perfect, most biologists are now prepared to accept that the endosymbiotic theory provides at least a partial explanation for the evolution of the eukaryotic cell from a prokaryotic ancestor. Unfortunately, living forms having a cellular organization intermediate between prokaryotes and eukaryotes are rare. Some primitive protists possess a nucleus but lack mitochondria and other typical eukaryotic organelles. They also have the prokaryotic type of ribosomes. These organisms are all intracellular parasites and they include Microspora, an organism that infects AIDS patients.
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Thu Mar 29, 2012 11:01 pm
