Pseudogene
From EvoWiki
A pseudogene is a non-coding section of DNA whose sequence resembles that of a functional gene, in that it has sequences homologous (similar) with those of functional genes, such as promotors and introns/exons, but which lacks the signals which allow it to be expressed. In the past the definition of pseudogene has been ambigious, with questions such as whether genes which produce RNA, but which are not translated into polypeptides (enzymes and proteins) should be classified as pseudogenes, but there is now a widespread concensus that "pseudogene" means only those "genes" which are not transcribed at all.
Pseudogenes are "Junk DNA" and probably have no significant short term selective advantages or disadvantages. There are a number of ways pseudogenes could form:
- Silencing of gene following loss of function by mutation.
- Silencing of gene following redundancy caused by changes in the environment in which the population lives.
- Silencing of one of two gene copies following a duplication of a section of DNA.
Steve Jones (1993) uses the analogy that genes are factories on an industrial estate, manufacturing proteins. Pseudogenes are the derelict factories which produced products no-longer needed, falling into further disrepair as mutations build up.
Pseudogenes as molecular clocks
Mutations tend to build up at a steady rate which geneticists can estimate. Coding DNA is subject to natural selection, so only a few mutations in such sections survive (coding DNA evolves at less than a tenth of the rate of non-coding DNA (Russel 2003)). There is nothing to stop mutations building up in pseudogenes, however, so these can be used as a "molecular clock" in phylogenetics. As two populations or species diverge their Junk DNA will acumulate mutations at a steady rate, which can be used to estimate the date at which the two populations split.
The rate of molecular evolution (r) is found by dividing the number of substitution mutations (K) by the time since divergence doubled (2T):
- r = K/(2T)
References
- Jones, S., 1993. The Language Of The Genes, London: HarperCollins.
- Russel, P.J., 2003. iGenetics, San Francisco: Benjamin Cummins, Ch 24.
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This page is part of the EvoWiki encyclopedia of genetics and molecular biology. Topics: Genetics - Transmission genetics - Molecular genetics - Population genetics - Quantitative genetics - Molecular biology - Genomics |
