Pseudogene
From Free net encyclopedia
Current revision
A pseudogene is a nucleotide sequences that is similar to a normal gene, but does not produce a functional final product.
Several scenarios have been proposed under which a pseudogene might arise:
- Fragments of the mRNA transcript of a gene may be spontaneously reverse transcribed and inserted into chromosomal DNA (called retrotransposition). These pseudogenes are called processed. Since these pseudogenes lack the promoters of normal genes, they are not normally expressed.
- A gene duplication event may mean that a genome has two copies of a gene when it only requires one. Deactivating mutations in one copy of the gene would then not be selected against (and might even have some selective advantage to becoming deactivated). In addition, the duplication event may not have been complete, so they might have incomplete promoters. These pseudogenes are called duplicated or non-processed.
- A gene may become nonfunctional or deactivated if such a mutation becomes fixed in the population. This can occur through normal means, such as natural selection or genetic drift. This is the same mechanism by which non-processed genes become deactivated.
Pseudogenes can complicate molecular genetic studies. For example, a researcher who wants to amplify a gene by PCR may simultaneously amplify a pseudogene that shares similar sequences. This is known as PCR bias or amplification bias. Similarly, pseudogenes are sometimes annotated as genes in genome sequences.
As is frequently the case in molecular biology, one can find unusual examples that challenge any simple definition of a term, and pseudogene is no exception. There is some difference among geneticists concerning the nature of the end product. If the end product is required to be a protein, then some pseudogenes can function as an RNA (see, for example, Hirotsune et al (2003) who discovered a sequence in the human genome that was identified as a pseudogene but apparently has a regulatory function for the homologous protein-coding gene). However, this definition does not allow for tRNA or rRNA pseudogenes, as other geneticists use the term.
References
- Hirotsune S, Yoshida N, Chen A, Garrett L, Sugiyama F, Takahashi S, Yagami K, Wynshaw-Boris A, Yoshiki A. (2003), "An expressed pseudogene regulates the messenger-RNA stability of its homologous coding gene.", Nature, 423:91-96. (A functional pseudogene.) In PubMed