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, the biological unit of inheritance, are present in all living organisms. The majority of the time these genes are silent; they are not expressed. At certain times, however, genes are switched on to produce their particular product. This is called gene expression. Different genes have different triggers to activate them and then to subsequently switch them off. For some genes, such as developmental genes, expression occurs only once in a life time, even though the genes are present at all times within the body.
When DNA is folded to take up the least amount of room, the genes are inaccessible and it is impossible for the genes to be expressed. For gene expression, enzymes are used to unfold the DNA to allow transcription and translation.
Gene expression can be switched on by external stimulus, and once the product has been manufactured the genes are then deactivated. For example, in certain species of bacteria the enzyme B galactosidase is only produced when lactose is present in the growth medium. The enzyme is basically produced to order. Some genes, which are present in high copy number, are present as such because their expression is required in high quantities. This allows for a rapid expression and production of product. The current hypothesis of how the gene expression is controlled is called the Jacob-Monod Hypothesis.