The final method of changing the DNA in a genome that we will consider is transposition, which is the movement of DNA from one location to another. Segments of DNA with this ability to move are called transposable elements. Transposable elements were formerly thought to be found only in a few species, but now they are recognized as components of the genomes of virtually all species.
- 9.1: Transposable Elements (Transposons)
- Transposable elements (both active and inactive) occupy approximately half the human genome and a substantially greater fraction of some plant genomes! These movable elements are ubiquitous in the biosphere, and are highly successful in propagating themselves. We now realize that some transposable elements are also viruses, for instance, some retroviruses can integrate into a host genome to form endogenous retroviruses.
- 9.2: Are Transposons Parasites or Symbionts?
- Do the transposable elements confer some selective advantage on the "host"? Or are they merely parasitic or "selfish," existing only to increase the number of copies of the element? This critical issue is a continuing controversy.
- 9.5: Transposition occurs by Insertion into Staggered Breaks
- A common property of virtually all transposable elements is that they move by inserting into a staggered break in a chromosome, i.e. one strand is slightly longer than the other at the break. The first indication of this was the observation that the same short DNA sequence is found on each side of a transposable element. The sequence within these flanking direct repeats is distinctive for each copy of the transposable element, but the size is characteristic of a particular family of elements.
- 9.6: Classes of Transposable Elements
- The two major classes of transposable elements are defined by the intermediates in the transposition process. One class moves by DNA intermediates, using transposases and DNA polymerases to catalyze transposition. The other class moves by RNA intermediates, using RNA polymerase, endonucleases and reverse transcriptase to catalyze the process. Both classes are abundant in many species, but some groups of organisms have a preponderance of one or the other.
- Mechanism of Retrotransposition
- Although the mechanism of retrotransposition is not completely understood, it is clear that at least two enzymatic activities are utilized. One is an integrase, which is an endonuclease that cleaves at the site of integration to generate a staggered break. The other is RNA-dependent DNA polymerase, also called reverse transcriptase. These activities are encoded in some autonomous retrotransposons, including both LTR-retrotransposons and non-LTR-retrotransposons.
- Unstable Alleles
- The insertion of a controlling element can generate an unstable allele of a locus, designated mutable. This instability can be seen both in somatic and in germline tissues. The instability can result from reversion of a mutation, due to the excision and transposition of the controlling element. After excision and re-integration, the transposable element can alter the expression of a gene at the new location. This new phenotype indicated that the element was mobile.
- Shapiro, J. A. (editor) (1983) Mobile Genetic Elements (Academic Press, Inc., New York).
- Fedoroff, N. and Botstein, D. (1992) The Dynamic Genome: Barbara McClintock’s Ideas in the Century of Genetics (Cold Spring Harbor Press, Plainview, NY).
- McClintock, B. (1952) Chromosome organization and genic expression. Cold Spring Harbor Symposium on Quantitative Biology 16: 13-47.
- Fedoroff, N., Wessler, S., and Shure, M. (1983) Isolation of the transposable maize controlling elements Ac and Ds. Cell 35:235-242.Boeke, J.D., Garfinkel, D.J., Styles, C.A., and Fink G.R. (1985) Ty elements transpose through an RNA intermediate. Cell 40:491-500
- Kazazian, H.H. Jr, Wong, C., Youssoufian, H., Scott, A.F., Phillips, D.G., and Antonarakis, S.E. (1988) Haemophilia A resulting from de novo insertion of L1 sequences represents a novel mechanism for mutation in man. Nature 332:164-166.
- International Human Genome Sequencing Consortium (2001) Initial sequencing and analysis of the human genome. Nature 409: 860-921. The material from pages 879-889 covers human repeats and transposable elements.