Author: Shenzhou

Reviewer: Weiming

  Transposons, also known as jumping genes, were first described in 1947 by the renowned American geneticist Barbara McClintock while she was studying the unstable inheritance of variegation in the maize aleurone layer. They revealed a new mechanism by which an intact genetic unit can move from one location to another within a genome. The phenomenon was analyzed further in a 2002 paper by Fu H. and Dooner H. K., which showed that, at the molecular level, every organism is a product of genetic recombination: new sequences continually arise through mutation, while existing sequences evolve through exchange, insertion, and rearrangement.

  A genetic unit capable of this movement is called a “transposable element,” or transposon. Transposition differs markedly in both concept and mechanism from homologous recombination and site-specific recombination, the latter being a characteristic mode of recombination in prokaryotes. Movement of chromosomal fragments through exchange, inversion, translocation, and related events depends on the RecA, RecB, and RecD recombination proteins. The sites and transferred fragments vary, and ultraviolet radiation or chemical mutagens can raise the frequency of these events. Transposon-mediated recombination, by contrast, depends on a transposase and inverted-repeat (IR) sequences and is not promoted by mutagens. A transposition site does not require sequence homology, although transposons do favor certain target sequences. Insertion and reversion mutations can therefore occur at relatively high frequencies. Because a discrete genetic segment moves from one site to another, the independent structural unit responsible is aptly called a “jumping gene.”

  In 1947, while investigating the genetic basis of variegated maize aleurone and plant pigmentation, McClintock found that variegation was often accompanied by the characteristic cytogenetic structure of a chromosome breakage–fusion–bridge cycle.

  She proposed that chromosome breakage in maize was caused by a dissociation factor called Ds. In a heterozygote, Ds lay between the centromere and certain dominant genes on one homologous chromosome; the other homolog lacked Ds and carried the recessive alleles. Breakage at Ds produced an acentric fragment, which was lost during mitosis. The resulting daughter cells therefore retained only the recessive genes on the intact homolog. The centromere-bearing remainder of the broken chromosome acquired sticky ends at the Ds break. After replication, those ends fused, producing a chromosome with two centromeres. At the next division, spindle fibers pulled the centromeres toward opposite poles, breaking the chromosome again. Repetition of this process created the breakage–fusion–bridge cycle.

  When transposition occurs during the growth and development of a maize kernel, spots appear in the aleurone layer. The earlier it occurs, the larger the spots; the more frequently it occurs, the more numerous they become. McClintock also showed that Ds does not insert autonomously but is controlled by an activator element, Ac. If Ac causes Ds to insert into an aleurone pigment gene, changing it into another allele, variegation appears. A later Ac-regulated jump by Ds can restore the original allele—a reversion mutation—giving the aleurone spots their unstable inheritance.