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Pseudogenes are dysfunctional relatives of genes that have lost their gene expression in the cell or their ability to code protein. Pseudogenes often result from the accumulation of multiple mutations within a gene whose product is not required for the survival of the organism. Although not protein-coding, the DNA of pseudogenes may be functional, similar to other kinds of non-coding DNA which can have a regulatory role. Although some do not have introns or promoters (these pseudogenes are copied from mRNA and incorporated into the chromosome and are called processed pseudogenes), most have some gene-like features such as promoters, CpG islands, and splice sites. They are different from normal genes due to a lack of protein-coding ability resulting from a variety of disabling mutations (e.g. premature stop codons or frameshifts), a lack of transcription, or their inability to encode RNA (such as with rRNA pseudogenes). The term was coined in 1977 by Jacq ''et al.'' Because pseudogenes are generally thought of as the last stop for genomic material that is to be removed from the genome,〔 they are often labeled as junk DNA. We can define a pseudogene operationally as a fragment of nucleotide sequence that resembles a known protein's domains but with stop codons or frameshifts mid-domain. Nonetheless, pseudogenes contain fascinating biological and evolutionary histories within their sequences. This is due to a pseudogene's shared ancestry with a functional gene: in the same way that Darwin thought of two species as possibly having a shared common ancestry followed by millions of years of evolutionary divergence (see speciation), a pseudogene and its associated functional gene also share a common ancestor and have diverged as separate genetic entities over millions of years. == Properties == Pseudogenes are characterized by a combination of homology to a known gene and nonfunctionality. That is, although every pseudogene has a DNA sequence that is similar to some functional gene, they are nonetheless unable to produce functional final protein products. Pseudogenes are sometimes difficult to identify and characterize in genomes, because the two requirements of homology and nonfunctionality are usually implied through sequence alignments rather than biologically proven. # Homology is implied by sequence identity between the DNA sequences of the pseudogene and parent gene. After aligning the two sequences, the percentage of identical base pairs is computed. A high sequence identity (usually between 40% and 100%) means that it is highly likely that these two sequences diverged from a common ancestral sequence (are homologous), and highly unlikely that these two sequences have evolved independently (see Convergent evolution). # Nonfunctionality can manifest itself in many ways. Normally, a gene must go through several steps to a fully functional protein: transcription, pre-mRNA processing, translation, and protein folding are all required parts of this process. If any of these steps fails, then the sequence may be considered nonfunctional. In high-throughput pseudogene identification, the most commonly identified disablements are premature stop codons and frameshifts, which almost universally prevent the translation of a functional protein product. Pseudogenes for RNA genes are usually more difficult to discover as they do not need to be translated and thus do not have "reading frames". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「pseudogene」の詳細全文を読む スポンサード リンク
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