Transposable Elements for Genetics Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Eukaryotic Elements

As mentioned earlier, eukaryotic cells can contain DNA elements that transpose on their own. Agood example of this type of element is the Ac-Ds system in maize.

EXAMPLE 11.8 Mobile genetic elements ("jumping genes") were first discovered in maize by Barbara McClintock in the 1950s. Insertion of the controlling element Ds into or adjacent to a locus governing kernel color inhibits the production of color and results in a colorless phenotype. Excision of Ds reverses the effect and produces colored spots on a colorless background. The Ds elements occur in different sizes as deleted forms of a larger complete element called Ac. The Ds elements are non-autonomous because they remain stationary unless an Ac element is also present, whereas Ac elements are autonomous because they can move independently. Both Ac and Ds elements have perfect inverted repeats of 11 bp at their termini, flanked by 6- to 8-bp direct repeats of the target site. Thus, Ac and Ds are transposons. Ac need not be adjacent to Ds or even on the same chromosome in order to activate Ds. When Ds is so activated, it can alter the level of expression of neighboring genes, the structure of the gene product, or the time of development when the gene expresses itself, as a consequence of nucleotide changes inside or outside a given gene. An activated Ds element can also cause chromosomal breakage, which can yield deletions or generate a bridge-breakage-fusion-bridge cycle. Several other systems like the Ac / Ds system are now known in maize. Each has a target gene that is inactivated by insertion of a receptor element into it, and a distant regulator element that is responsible for the mutational instability of the locus. The receptor and regulator elements are both considered to be controlling elements of the target gene.

The other type of element found in eukaryotic systems is the retrotransposon. The retrotransposition process begins with transcription of DNA into mRNA. The RNA is then copied by the action of a reverse transcriptase enzyme into a DNA copy (cDNA). This cDNA then integrates into a new region of the genome. The reverse transcriptase is generally one of the genes encoded by the retrotransposon. Another commonly encoded gene is integrase, an enzyme required for integration of the cDNA into the genome. Retrotransposons exist in high copy number in mammalian genomes (up to 500,000 elements making up as much as 40% of the genome). They can be short (200–200 bp) or long (3,000–8,000 bp) DNA segments. Short elements are called SINEs, for short interspersed elements and the longer ones are termed LINEs for long interspersed elements.

EXAMPLE 11.9 In a human genome, there exist about 300,000 members of a SINE sequence (approximately 300 bp) that is cut by a base-specific DNase called AluI. Members of this Alu family are related, but not identical in base sequence. Each member is flanked by direct repeats. Although transposition has not been observed for any member, the Alu family is thought to have evolved from a DNA copy of an RNA molecule that plays a role in protein synthesis. Since no function, essential or otherwise, has been attributed to this family, it may represent an example of "selfish DNA" whose function is to make copies of it self.

The most common LINE in mammals is the L1 element. The Ty element in yeast and the copia element in Drosophila are other examples of retrotransposons. Many of these elements contain genes or processed pseudogenes related to cellular and retroviral genes. Pseudogenes are nonfunctional genes with sequence similarity to a functional gene found elsewhere in the genome of an organism.

Practice problems for these concepts can be found at:

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