Barbara McClintock discovered the first mobile elements while doing classical genetic experiments in maize (corn) during the 1940s. She characterized genetic entities that could move into and back out of genes, changing the phenotypes of corn kernels. Her theories were very controversial until similar mobile elements were discovered in bacteria, where they were characterized as specific DNA sequences, and the molecular basis of their transposition was deciphered.

As research on mobile elements progressed, they were found to fall into two categories: (1) those that transpose directly as DNA and (2) those that transpose via an RNA intermediate transcribed from the mobile element by an RNA polymerase and then converted back into double-stranded DNA by a reverse transcriptase (Figure 8-8). Mobile elements that transpose directly as DNA are generally referred to as DNA transposons, or simply transposons. Eukaryotic DNA transposons excise themselves from one place in the genome, leaving that site and moving to another. Mobile elements that transpose to new sites in the genome via an RNA intermediate are called retrotransposons. Retrotransposons make an RNA copy of themselves and introduce this new copy into another site in the genome, while also remaining at their original location. The movement of retrotransposons is analogous to the infectious process of retroviruses (see Figure 5-48). Indeed, retroviruses can be thought of as retrotransposons that evolved genes encoding viral coats that allowed them to transpose between cells. Retrotransposons can be further classified on the basis of their specific mechanism of transposition. To summarize, DNA transposons can be thought of as transposing by a “cut-and-paste” mechanism, while retrotransposons move by a “copy-and-paste” mechanism in which the copy is an RNA intermediate.

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