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Classifying Living Things
| Article
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21693 |
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Section : |
NATURAL SCIENCE
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| Issue
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1 / 2002 |
188 Words |
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Dinshaw K. Dadachanji is a science editor for The World & I. |
It was once thought that all life-forms could be classified as either plants or animals. By the early 1970s, scientists had expanded the classification to five kingdoms: bacteria, plants, animals, protists, and fungi. It was further recognized that the cells of organisms in the latter four groups have certain common features, particularly nuclei, and the organisms were called eukaryotes. On the other hand, bacterial cells lack nuclei, so bacteria were said to be prokaryotes.
In the late 1970s, Carl Woese and his research team realized that there are two distinct groups of prokaryotes, based on differences in their DNA sequences. Woese therefore proposed that living organisms be classified under three domains: Eubacteria, Archaebacteria, and Eukaryota. These domains were later renamed Bacteria, Archaea, and Eukarya. All eukaryotes are collectively placed under Eukarya.
Viruses are microbial entities that sit on the fence between living and nonliving matter. They have no cellular structure and must therefore infect the cells of a host to grow and replicate. Outside the host cell, they seem inanimate. Viruses that infect bacteria are known as bacteriophages; those that infect eukaryotes are called eukaryotic viruses.--D.K.D.
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