Mobile DNA: An introduction

The once revolutionary idea that genomes contain segments of mobile DNA was actually first conceptualized before the discovery of the structure of DNA. Mobile DNA was first suggested by McClintock when she described "controlling elements" in maize. A notable feature of the work presented in this volume is that our understanding of many of these recombination reactions has moved to the biochemical and structural level. It is appropriate to begin a discussion of site-specific recombination reactions by considering conservative site-specific recombination. Genetic studies of these reactions in the 1950s provided one of the earliest examples of genome modification by a mobile DNA. Transposition is the recombination reaction that mediates the movement of discrete DNA segments between many nonhomologous sites. These segments are variously called insertion sequences, transposons, and transposable elements. There are two large classes of transposable elements: (i) those in which DNA is the actual substrate for recombination and (ii) those in which RNA is the actual substrate for recombination. Several recombination systems mediate the alternate expression of multiple alleles of a gene by moving alternative gene copies or gene segments from a "silent" position to an active "expression site". The continued study of genomes will lead us to more mobile DNAs to identify and understand, and further investigation into mobile DNAs will help us to understand the structure and function of genomes.