The following sections of this BookRags Literature Study Guide is offprint from Gale's For Students Series: Presenting Analysis, Context, and Criticism on Commonly Studied Works: Introduction, Author Biography, Plot Summary, Characters, Themes, Style, Historical Context, Critical Overview, Criticism and Critical Essays, Media Adaptations, Topics for Further Study, Compare & Contrast, What Do I Read Next?, For Further Study, and Sources.
(c)1998-2002; (c)2002 by Gale. Gale is an imprint of The Gale Group, Inc., a division of Thomson Learning, Inc. Gale and Design and Thomson Learning are trademarks used herein under license.
The following sections, if they exist, are offprint from Beacham's Encyclopedia of Popular Fiction: "Social Concerns", "Thematic Overview", "Techniques", "Literary Precedents", "Key Questions", "Related Titles", "Adaptations", "Related Web Sites". (c)1994-2005, by Walton Beacham.
The following sections, if they exist, are offprint from Beacham's Guide to Literature for Young Adults: "About the Author", "Overview", "Setting", "Literary Qualities", "Social Sensitivity", "Topics for Discussion", "Ideas for Reports and Papers". (c)1994-2005, by Walton Beacham.
All other sections in this Literature Study Guide are owned and copyrighted by BookRags, Inc.
Genes that code for vital cellular functions are highly conserved through evolutionary time, and because even these genes experience random changes over time, the comparison of such genes allows the relatedness of different organisms to be assessed. American microbiologist Carl Woese and his colleagues obtained sequences of the genes coding for RNA in the subunit of the ribosome from different organisms to argue that life on Earth is comprised of three primary groups, or domains. These domains are the Eukarya (which include humans), Bacteria, and Archaea.
While Archae are microorganisms, they are no more related to bacteria than to eukaryotes. They share some traits with bacteria, such as having a single, circular molecule of DNA, the presence of more mobile pieces of genetic material called plasmids, similar enzymes for producing copies of DNA. However, their method of protein production and organization of their genetic material bears more similarity to eukaryotic cells.
The three domains are thought to have diverged from one another from an extinct or as yet undiscovered ancestral line. The archae and eukarya may have branched off from a common ancestral line more recently than the divergence of these two groups from bacteria. However, this view remains controversial and provisional.
The domain Archae includes a relatively small number of microoganisms. They inhabit environments which are too harsh for other microbes. Such environments include hot, molten vents at the bottom of the ocean, the highly salt water of the Great Salt Lake and the Dead Sea, and in the hot sulfurous springs of Yellowstone National Park. Very recently, it has been shown that two specific archael groups, pelagic euryarchaeota and pelagic crenarchaeota are one of the ocean's dominant cell types. Their dominance suggests that they have a fundamentally important function in that ecosystem.