All living things are classified into kingdoms. Living organisms are classified based on similarities within their DNA, structure, reproduction, and appearance. Having this kind of grouping system allows scientists to study one or more of the 1.5 million species of organisms that have been discovered so far. This also allows scientists to discover one or more of the millions of species that have yet to be discovered.

Currently there are five widely accepted kingdoms of which all living things are classified: monera, protists, fungi, plants, and animals. All organisms within each kingdom share characteristics that are unique to that particular group. Most monera are unicellular or one-celled and contain no nucleus. Protists are also unicellular but they contain a nucleus. Fungi can be unicellular or multicellular (containing many cells) and feed off other organisms by absorbing nutrients. Plants are multicellular, make their own food, and are immobile. Animals are multicellular, depend on other animals and plants for food, and are mobile.

Many scientists accept that there are actually 6 kingdoms. The Monera Kingdom can be divided into the bacteria kingdom and the archaebacteria kingdom. The reasoning behind this distinction is the fact that both groups have a different genetic make-up from each other. This is a major difference. Because of the genetic difference, bacteria and archaebacteria have different structures and react differently to certain antibiotics (medicines used to kill bacteria and archaebacteria).

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