14: Repetitive DNA - A Eukaryotic Genomic Phenomenon
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- 14.1: Introduction
- Because of their small size, bacterial genomes have few repetitive DNA sequences. In contrast, repetitive DNA sequences make up a large part of a eukaryotic genome. Much of this repeated DNA consists of identical or nearly identical sequences of varying length repeated many times in a genome. Examples include satellite DNA (minisatellite and microsatellite DNA) and transposons, or transposable elements. Here we look at experiments that first revealed the existence and proportion of repeated DNA
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- 14.2: The Complexity of Genomic DNA
- By the 1960s, when Roy Britten and Eric Davidson were studying eukaryotic gene regulation, they knew that there was more than enough DNA to account for the genes needed to encode an organism. It was also likely that DNA was more structurally complex than originally thought. They knew that cesium chloride (CsCl) density gradient centrifugation separated molecules based on differences in density and that fragmented DNA would separate into a main and a minor band of different density in centrifuge
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- 14.3: The 'Jumping Genes' of Maize
- Barbara McClintock’s report that bits of DNA could jump around and integrate themselves into new loci in DNA was so dramatic and arcane that many thought the phenomenon was either a one-off, or not real! Only with the subsequent discovery of transposons in bacteria (and in other eukaryotes) were McClintock’s jumping genes finally recognized for what they were!
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- 14.4: Transposons Since McClintock
- Transposons exist everywhere we look in prokaryotes and account for much of eukaryotic repetitive DNA. As such, they can be a large proportion of eukaryotic genomes, including some that no longer even transpose. Transposons were once considered useless or junk DNA, with no obvious function…, or selfish genes with no other purpose than selfreplication. But in light of some new evidence, perhaps not!
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- 14.5: On the Evolution of Transposons, Genes, and Genomes
- We noted that transposons in bacteria carry antibiotic resistance genes, a clear example of benefits of transposition in prokaryotes. Of course, prokaryotic genomes are small, as is the typical bacterial transposon load. Yeast species also have low transposon load. But, what can we make of the high transposon load in eukaryotes?
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- 14.6: Roles of Transposition in Evolution and Diversity
- A role for unequal recombination in moving exons in and out of different eukaryotic split genes was described earlier. This kind of exon shuffling could happen when short DNA sequences in two different introns misalign during meiotic synapsis, allowing for unequal crossing over. Expression of a gene with a ‘new’ exon produces a protein with a new domain and a new activity. If the event is not harmful, diversity is increased!
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- 14.7: Coping with the Dangers of Rampant Transposition
- Most organisms do not have the high transposon load that we have. For those like us, and given a general tendency of transposons to insert at random into new DNA loci, how come we exist at all? Isn’t the danger of transposition into essential gene sequences magnified by the possibility of multiple simultaneous transpositions of elements generated by cut-and-paste and especially replicative mechanisms? Indeed, transposons have been found in genes that are inactive as a result.
Thumbnail: Maize grains (Hopi Blue) with pigmentation modified by the action of transposons. (CC BY-SA 3.0 Unported; Abrahami and modified by LibreTexts via Wikipedia )