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.
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During the S-phase of the cell cycle, DNA is replicated and each chromosome is present in a duplicated state with the two genetically identical chromatids joined together at the centromere. These two sister chromatids are readily apparent in late prophase or early metaphase of mitosis.
Sister chromatid exchange (SCE) is the process wherein the two sister chromatids break and rejoin with one another, physically switching positions on the chromosome. Because the exchanges occur with tremendous precision with respect to the DNA sequence, and the sister chromatids are genetically identical, no information is altered during the exchange. Such exchanges are natural events during cell replication with each cell typically undergoing three to four SCEs during each replication cycle.
SCEs can be visualized by growing cells in a medium, which contains the DNA base analog, bromodeoxyuridine (BrdUrd). BrdUrd closely resembles thymidine, and gets incorporated into the growing DNA strand during replication. Because replication of DNA is semiconservative, all of the BrdUrd is taken up into the newly synthesized strand, with the template strands remaining free of BrdUrd. During a second round of growth in BrdUrd medium, the two sister chromatids differ in the amount of BrdUrd present. The sister chromatid which has the original template strand of DNA has one strand of normal DNA and one which contains BrdUrd. The other sister chromatid has BrdUrd incorporated into both strands. With careful chemical and ultraviolet light treatment, the two sister chromatids can be distinguished from each other. SCEs can then be identified by differences in staining patterns.
SCE is useful for genetic toxicology studies as chemicals, that cause mutations, will often increase the frequency of SCEs. SCE may be used as a diagnostic test for the rare genetic disease called Bloom syndrome.