Figure 1501

Transposable Elements are Never Free

Transposable elements are stretches of DNA able to move from one position to another, but are always found within a DNA molecule such as a bacterial plasmid (top) or a eukaryotic chromosome (bottom). Transposons do not contain their own origin of replication, but rely on the host DNA to provide this feature.

Origin of replication

Origin of replication

Plasmid

Origin of replication

Chromosome

Origin of replication

Chromosome

Chromosomes, plasmids and virus genomes are capable of self-replication, transposons are not.

As noted in Chapter 5, any molecule of DNA that possesses its own origin of replication is known as a replicon. Chromosomes, plasmids and virus genomes are repli-cons and may be regarded as self-replicating. In contrast, transposons lack a replication origin of their own and are not replicons. They can only be replicated by integrating themselves into other molecules of DNA, such as chromosomes, plasmids or virus DNA. As long as the DNA molecule of which the transposon is part gets replicated, the transposon will also be replicated. If the transposon inserts itself into a DNA molecule with no future, the transposon "dies" with it.

Transposable elements are classified based on their mechanism of movement. The major division is between those that move via an RNA intermediate and need reverse transcriptase to generate this, and those with a DNA-based mechanism. The DNA based transposons are subdivided into two main groups according to whether a new copy is generated during transposition (complex or replicative transposition) or whether the original copy moves, leaving a gap in the DNA in its previous location (conservative or "cut-and-paste" transposition). Table 15.01 summarizes the main groups of transposable elements whose properties will be discussed in more detail below.

The enzymes responsible for transposition must recognize repeated sequences at each end of the transposon.

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