Multiple defective copies of transposons are often found in higher organisms. They rely on an intact copy to help them move.
Conservative transposons of the mariner family are widespread in eukaryotes.
The Ac/Ds family of transposons in corn is simple and conservative. Family members leave behind double stranded gaps in the DNA when they move. They have inverted terminal repeats of 11 base pairs and insert at an 8 bp target sequence. The Ac element is 4,500 bp long and is a fully functional transposon with the ability to move itself. The Ds elements vary in size and are defective. They are derived from Ac by deletion of all or part of the transposase gene and they cannot move by themselves. If a cell contains an Ac element anywhere in its DNA, then the transposase enzyme made by Ac can also move any Ds element. Therefore to remain mobile, the Ds elements must keep the inverted repeats, otherwise the Ac transposase will not recognize them (Fig. 15.14). The Ac and Ds elements do not need to be on the same chromosome for transposition to occur.
If a Ds element has been inserted into the gene for purple kernels of corn, the gene is disrupted and the kernels are white. If there is no Ac element in any of the cells of the entire corn kernel the white color is stably inherited. If an Ac element is also present in some of the cells of the corn kernel, it may move the Ds element. The cells in which both elements are present return to the original purple color. All the daughter cells of these will also be purple. Eventually, a patch of purple will appear on the kernel as it grows. If the transposition occurs when the kernel is just beginning to develop, a large patch of purple will appear, and if the tranposition occurs when the kernel is almost fully developed the patch of purple will be very small. This produces a mottled kernel of corn (Fig. 15.15).
Animals and plants frequently contain transposon families with both active and defective members. These may vary greatly in their overall lengths. Not only are there defective members that need help to move, but we also find totally inactive transposons. These have suffered mutations in their terminal repeats which makes them unrecognizable by transposase and therefore immobile.
The most widely distributed transposons in higher organisms are those of the Tc1/mariner family. The first members to be found were Tc1, from the nematode
Ac element Intact and active version of a transposon found in maize
Ds elements Defective version of a transposon found in maize;cannot move alone but needs the Ac element to provide transposase Tc1 element Transposon Caenorhabditis 1. A transposon of the mariner family found in the nematode Caenorhabditis
FIGURE 15.15 Movement of Ds Element Gives Mottled Corn
A: The gene for purple color in maize produces a purple kernel of corn.
B: If the gene for purple color is disrupted by a Ds element, the kernel is white.
C: If both the Ac and Ds element are present in the same cell, transposase from the Ac element moves the Ds element from the purple gene, returning the cell to the original purple color. As the cell divides, the purple color is inherited in the daughter cells, and a patch of purple forms on the white kernel. Random transposition events in several such cells results in a mottled kernel.
D: Photog raph of mottled corn cob showing reddish-purple streaks due to transposition of Ac/Ds. The patterns caused by transposition may be blotches, dots, irregular lines or streaks.
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