Pseudogenes

AbstractPseudogenes are nonfunctional genomic regions that arise mostly from the duplication of functional genes. Two types of pseudogenes are generally formed by independent mechanisms that are believed to have different implications in gene evolution.Nonprocessedpseudogenes arise after partial or complete segmental duplication of genes and subsequent loss of function by mutations.Processedpseudogenes, in contrast, are formed through the reverse transcription of mature RNAs and their integration back in the genome (retrotransposition). As pseudogenes share many sequence characteristic with functional genes, the identification and differentiation of pseudogenes is often difficult and prone to controversy. Nevertheless, a few sequence features that suggest the absence of functionality have been exploited to obtain pseudogene collections from different available genomes. In human and rodents, nearly 20 000 pseudogenes have been identified, most of which were found to have a retrotranspositional origin. In contrast, a much lower number of pseudogenes could be identified in other organisms, such as chicken and nonvertebrates. Despite the recent identification of large collections of pseudogenes, improvements in the distinction between functional and pseudogenic regions are still required in order to determine what the actual impact of these regions is in the evolution of the genomes.