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Modern Genetic Analysis Chapter 13. Transposable Genetic Elements
Introduction

Genetic studies of maize, by Barbara McClintock and by 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 (see page 427) 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 elements, which is formally most correct and embraces the entire family of types. The term transposition has long been used in genetics to describe the 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.

Transposable genetic elements seem to be able to move to new positions within the same chromosome or even to move to a different chromosome. The normal genetic role of these elements is not known with certainty. They have been detected genetically through the abnormalities that they produce in the activities and structures of the genes near the sites to which they move. A variety of physical techniques have been used to detect them as well. Transposable genetic elements have been found in phages, bacteria, fungi, higher plants, viruses, and insects.

Although transposable elements were first detected in eukaryotes, the molecular nature of transposable genetic elements was first understood in bacteria and phages. Therefore, we shall begin with the information derived from the original studies in these prokaryotes.

© 1999 by W. H. Freeman and Company. All rights reserved.