Pseudogenes are junk DNA. Pseudogenes traditionally were defined as genetic loci bearing close structural resemblance to known functional genes but that themselves are non-functional due to genetic alterations such as additions, deletions, or nucleotide substitutions. They represent the descendents of functional genes from which they originated via various gene duplication processes. Pseudogenes come in two basic types according to their precise mode of origin: processed pseudogenes that lack introns because they arise when functional messenger RNA is retrotranspositionally inserted into the genome; and non-processed pseudogenes that are the evolutionary remnants of tandemly duplicated genes that became disabled by post-formational mutations. Ever since their discovery in the 1970s, pseudogenes were thought to provide quintessential examples of useless genomic junk. This intracellular “trash” also appeared to be pervasive. In the human genome, for example, about 12,000 DNA sequences were interpreted to show evidence of being pseudogenes (compared to about 25,000 functional protein-coding loci).
pseudogenes; genetic role; potogenes
Pseudogenes are junk DNA. Pseudogenes traditionally were defined as genetic loci bearing close structural resemblance to known functional genes but that themselves are non-functional due to genetic alterations such as additions, deletions, or nucleotide substitutions. They represent the descendents of functional genes from which they originated via various gene duplication processes (see Chapter 39). Pseudogenes come in two basic types according to their precise mode of origin: processed pseudogenes that lack introns (see Chapter 49) because they arise when functional messenger RNA is retrotranspositionally inserted into the genome (see Chapter 42); and non-processed pseudogenes that are the evolutionary remnants of tandemly duplicated genes that became disabled by post-formational mutations. Ever since their discovery in the 1970s, pseudogenes were thought to provide quintessential examples of useless genomic junk. This intracellular “trash” also appeared to be pervasive. In the human genome, for example, about 12,000 DNA sequences were interpreted to show evidence of being pseudogenes (compared to about 25,000 functional protein-coding loci).
Beginning in the 1980s, examples gradually came to light in which some “pseudogenes” played active cellular roles, such as in regulating gene expression (see Chapter 41) or generating useful genetic diversity. This new worldview – that many pseudogenes have utilitarian genomic roles – reached a crescendo in 2003 with the publication of an influential review touting the concept that pseudogenes should be considered “potogenes”: DNA sequences with the evolutionary potential for becoming new functional loci.
This radical new paradigm is broadly important because it provides a fine example of the concept of evolutionary preadaptation or exaptation (see Chapter 52) at the molecular level. Evolutionary geneticists now appreciate that many otherwise puzzling features of the eukaryotic genome, such as mobile elements (see Chapter 24) remain present in genomes in part because they have been coopted to serve modern adaptive cellular roles that may be entirely different from their original evolutionary raison d’être.
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