Molecules of DNA may be partitioned between dividing cells or they may be transferred from one cell to another. But to a geneticist, the term "mobile DNA" has a specialized meaning. Mobile DNA refers to segments of double-stranded DNA that move as discrete units from place to place within other DNA molecules. [Certain mobile DNA elements may exist transiently as separate molecules, but they cannot function independently—e.g. the gene cassettes of integrons, discussed below.] Segments of mobile DNA may move from one site to another on the same larger DNA molecule or from one host DNA molecule to another. Some insert more or less at random whereas others can insert only at specific sequences on the host DNA molecule.
Although the DNA of certain viruses can insert itself into the chromosomes of the host cell, most mobile DNA consists of genetic elements known as transposons or transposable elements. They are also sometimes called "jumping genes" because they may hop around from place to place among the chromosomes. The process of jumping from one site to another is called transposition. Transposons are not merely dependent on a host cell like plasmids and viruses; they are dependent on a host DNA molecule! Transposons are always inserted into other DNA molecules so they are never free as separate molecules (Fig. 15.01).
gene creature Genetic entitiy that consists primarily of genetic information, sometimes with a protective covering, but without its own machinery to generate energy or replicate macromolecules genetic element Any molecule or segment of DNA or RNA that carries genetic information and acts as a heritable unit jumping gene Popular name for a transposable element mobile DNA Segment of DNA that moves from site to site within or between other molecules of DNA
transposable element A mobile segment of DNA that is always inserted in another, host molecule, of DNA. It has no origin of replication of its own and relies on the host DNA molecule for replication. Includes both DNA-based transposons and retrotransposons transposition The process by which a transposon moves from one host DNA molecule to another transposon Same as transposable element, although the term is usually restricted to DNA-based elements that do not use reverse transcriptase
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