遗传分析-第二十章Transposable Genetic Elements.docVIP

Chapter 20 Transposable Genetic ElementsKey Concepts A series of genetic elements can occasionally move, or transpose, from one position on a chromosome to another position on the same chromosome or on a different chromosome. In bacteria, insertion sequences, transposons, and phage mu are examples of transposable genetic elements. Transposable elements can mediate chromosomal rearrangements. In higher cells, transposable elements have been extensively characterized in yeast, Drosophila, and maize and in mammalian systems. In eukaryotes, some transposable elements utilize an RNA intermediate during transposition, whereas, in prokaryotes, transposition is exclusively at the DNA level. Introduction Starting in the 1930s, genetic studies of maize, undertaken independently by Barbara McClintock and Marcus Rhoades, yielded results that greatly upset the classical genetic picture of genes residing only at fixed loci on the main chromosome. The research literature began to carry reports suggesting the existence of genetic elements of the main chromosomes that can somehow mobilize themselves and move from one location to another. These findings were viewed with skepticism for many years, but it is now clear that such mobile elements are widespread in nature. A variety of colorful names (some of which help to describe their respective properties) have been applied to these genetic elements: controlling elements, cassettes, jumping genes, roving genes, mobile genes, mobile genetic elements, and transposons. We choose the term transposable genetic element, which is formally most correct and embraces the entire family of types. The term transposition has long been used in genetics to describe transfer of chromosomal segments from one position to another in major structural rearrangements. In the present context, what is being transposed seems to be a gene or a small number of linked genes or a gene-sized fragment. Any genetic entity of this size can be called a genetic element