Most genes come in a single copy per haploid genome. The standard sentiment at that time was that genes are like individual beads successively strung along a chromosomal string. In other words, each gene was represented as a single copy per haploid eukaryotic genome.
repetitive sequences; reassociation kinetics
Most genes come in a single copy per haploid genome. The standard sentiment at that time was that genes are like individual beads successively strung along a chromosomal string (see Chapters 9 and 14). In other words, each gene was represented as a single copy per haploid eukaryotic genome.
During the 1950s and 1960s, a paradigm shift took place as biologists gradually accumulated evidence that much of the genome in any eukaryotic species consists of repetitive elements. No single discovery triggered this conceptual revolution, but a seminal paper by the geneticists Roy Britten and David Kohne in 1968 helped to crystallize and popularize this shifting attitude about the fundamental structure of eukaryotic genomes. Some of the earliest evidence for repetitive DNA came from DNA–DNA hybridization studies of various species in which it was noted that artificially dissociated strands of DNA reannealed in a test tube much faster than would be expected if each gene existed in only a single copy per genome. The reassociation kinetics instead implied that many DNA sequences in a genome must be moderately or highly repetitive.
Today, many categories of repetitive sequences are recognized, including simple or low-copy gene duplications, microsatellites (extensive arrays of individually very short tandem repeats), minisatellites (tandem arrays of somewhat longer repeats), and various categories of mobile elements that may be dispersed throughout the genome (see Chapter 24). Speculation about the functional significance of repetitive DNAs is equally diverse. On the positive side of the ledger, one common conceptual theme is that repetitiveness confers genetic redundancy and thereby facilitates the evolution of new operational capabilities in the host organism. Such enhanced cellular capacities might include the production in abundance of some much-needed protein or RNA product, novel or better opportunities to properly regulate a cell’s gene expression, architectural roles in chromatin structure, and many others. Another distinct possibility is that some classes of repetitive elements (notably jumping genes) are mostly genomic garbage, perhaps often serving no functional role beyond their own selfish proliferation (see Chapter 48). In truth, these two viewpoints on repetitive DNA (valuable resource versus trash) are not necessarily in evolutionary opposition. Even if many repetitive sequences proliferated as selfish or parasitic elements, they may often become evolutionarily coopted by the host genome to perform useful cellular functions (see Chapter 52).
1. Britten RJ, Kohne DE. Repeated sequences in DNA. Science. 1968;161:529–540.
2. Ohno S. Evolution by Gene Duplication New York, NY: Springer-Verlag; 1970.
3. Britten RJ, Davidson EH. Repetitive and non-repetitive DNA sequences and a speculation on the origins of evolutionary novelty. Q Rev Biol. 1971;46:111–138.
4. Goldstein DB, Schlötterer C. Microsatellites: Evolution and Applications Oxford, UK: Oxford University Press; 1999.
5. Shapiro JA, von Sternberg R. Why repetitive DNA is essential to genome function. Biol Rev Camb Philos Soc. 2005;80:227–250.
6. Lynch M. The Origins of Genome Architecture Sunderland, MA: Sinauer; 2007.
7. Fontdevila A. The Dynamic Genome: A Darwinian Approach Oxford, UK: Oxford University Press; 2011.