A pseudogene is a nucleotide sequences that is similar to a normal gene, but is not expressed as a functional protein.

Several scenarios have been proposed under which a pseudogene might arise:

1. 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. In addition, the duplication event may not have been complete, so they might have incomplete promoters. These pseudogenes are called duplicated.
2. 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 never expressed.
3. A gene may "die" during evolution if the selective pressure for its function is removed. For example, the environment in which a species lives could change such that the gene product is no longer necessary, or even harmful. Deactivating mutations in the gene would then no longer be selected against.

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. Similarly, pseudogenes are sometimes annotated as genes in genome sequences. Schematic of a region of a chromosome before and after a duplication event Gene duplication occurs when an error in DNA replication leads to the duplication of a region of DNA containing a (generally functional) gene. ... The life cycle of an mRNA in a eukaryotic cell. ... Reverse transcriptase is an enzyme used by all retroviruses and retrotransposons that transcribes the genetic information from the virus or retrotransposon from RNA into DNA, which can integrate into the host genome. ... Polymerase chain reaction (PCR) is a molecular biology technique for enzymatically replicating DNA without using a living organism, such as E. coli or yeast. ... In biology the genome of an organism is the whole hereditary information of an organism that is encoded in the DNA (or, for some viruses, RNA). ...

In some cases, the definition of "pseudogene" can become cloudy. For example, Hirotsune et al (2003) discovered a sequence that appeared to be a pseudogene in the sense that it did not code for a protein sequence, in contrast to a homologous gene that did code for a functional protein. However, the pseudogene is expressed and its RNA transcript regulates the activity of the homologous gene. Thus, with respect to protein-coding ability, this sequence is a pseudogene, but in a sense it is a functional non-coding RNA gene. A non-coding RNA (ncRNA) is any RNA molecule that functions without being translated into a protein. ... This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ...

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

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See also

• Junk DNA