Pseudogene

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Pseudogenes are gene-like structures present in an organism's genome that have lost the ability to code for proteins due to mutation. [1] They were first identified and dubbed in the late 1970s when researchers began finding non-coding regions in some organisms that were similar to actual coding genes in other organisms. [2] So far an estimated 19,000 pseudogenes have been identified in the human genome, this is almost equal to the total number of coding genes (21,000). [2] Humans have many pseudogenes including L-gulonolactone oxidase which is used to synthesize vitamin c. Research reports that this gene was inactivated in the common ancestor of all simians. [3]

Contents

Creationist Perspective

Creationist scientists assert that pseudogene analysis used to argue to validity of the theory of evolution is invalid. [4][5]

Finding pseudogenes and the Evolutionary Perspective

Pseudogenes have been identified in a wide range of organisms from bacteria to mice to humans, the total number of pseudogenes in a given genome is not predictable but specific pseudogenes are often compared across species to assert complex evolutionary relationships [2]

Pseudogenes are often difficult to parse from the large amount of non-coding base pairs in the genome. Convention requires two elements to be present to label a sequence a pseudogene. The first is homology which is the requirement that a sequence be demonstrated to descend from a functional copy of the gene and the second is non-functionality which is the requirement that the gene not code for a protein in the organism in question. [2]

Since all pseudogenes are asserted to be descended from a functioning gene the first step is to find the parent gene that it descended from. This is done by using computer programs to compare sequences of DNA across species. [2] This is a large computational problem but by keeping in mind the phylogenetic relationships between species the search time can be decreased by looking at species that share a more recent common ancestor.[6] Once a functioning copy of a gene is detected its sequence is compared to the pseudogene. A high correlation in base pairs is used to assign homology. Non-functionality can be demonstrated by attempting to transcribe the sequence in-vitro. [2]

Pseudogenes and neutral selection theory

Because pseudogenes do not code for a function many scientists have hypothesized that the accumulation of mutations would not be constrained by selection pressures.[1] This is known as neutral selection, and pseudogenes have been studied extensively to test various theories of neutral selection. [7] It has been determined that mutations fixate in pseudogene regions at about 30 percent higher than in coding regions of DNA. [7] Some theorist have argued that there maybe some selection pressure on pseudogenes (such as on genome size in general) so conclusions should be tempered.[1] Others have determined that base pair mutations are not completely random, favoring accumulation of guanine and cytosine. [7] Despite these findings research on pseudogenes still continues to be a productive avenue for exploring mutation and selection.

See also

References

  1. 1.0 1.1 1.2 Petrov, D.A, Hartl, D.L. (2000). Pseudogene evolution and natural selection for a compact genome. The American Genetic Association 91:221-227. [1]
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Gerstein, M, Zheng, D. (2006). The real life of pseudogenes. Scientific American 95:48-55. [2]
  3. http://www.cast.uark.edu/local/icaes/conferences/wburg/posters/kmilton/kmilton.html
  4. http://www.answersingenesis.org/tj/v14/i3/pseudogenes.asp
  5. http://www.answersingenesis.org/tj/v14/i3/pseudogenes_genomes.asp
  6. Bensasson, D., Zhang, D., Hartl, D., Hewitt, G. (2001). Mitochondrial pseudogense: evolution's misplaced witness. Trends in Ecology and Evolution 16: 314-321. [3]
  7. 7.0 7.1 7.2 Bustamante, C, Neilsen R, Hartl, D. (2002). A maximum likelihood method for analyzing pseudogene evolution: implications for silent site evolution in humans and rodents. 19:110-117. [4]

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