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Alternative splicing is a regulated process during gene expression that results in a single gene coding for multiple proteins. In this process, particular exons of a gene may be included within or excluded from the final, processed messenger RNA (mRNA) produced from that gene.〔 Consequently the proteins translated from alternatively spliced mRNAs will contain differences in their amino acid sequence and, often, in their biological functions (see Figure). Notably, alternative splicing allows the human genome to direct the synthesis of many more proteins than would be expected from its 20,000 protein-coding genes. Alternative splicing is sometimes termed ''differential splicing''. Alternative splicing occurs as a normal phenomenon in eukaryotes, where it greatly increases the biodiversity of proteins that can be encoded by the genome;〔 in humans, ~95% of multi-exonic genes are alternatively spliced.〔 There are numerous modes of alternative splicing observed, of which the most common is exon skipping. In this mode, a particular exon may be included in mRNAs under some conditions or in particular tissues, and omitted from the mRNA in others.〔 The production of alternatively spliced mRNAs is regulated by a system of trans-acting proteins that bind to cis-acting sites on the primary transcript itself. Such proteins include splicing activators that promote the usage of a particular splice site, and splicing repressors that reduce the usage of a particular site. Mechanisms of alternative splicing are highly variable, and new examples are constantly being found, particularly through the use of high-throughput techniques. Researchers hope to fully elucidate the regulatory systems involved in splicing, so that alternative splicing products from a given gene under particular conditions could be predicted by a "splicing code".〔〔 Abnormal variations in splicing are also implicated in disease; a large proportion of human genetic disorders result from splicing variants.〔 Abnormal splicing variants are also thought to contribute to the development of cancer.〔〔 ==Discovery== Alternative splicing was first observed in 1977. The Adenovirus produces five primary transcripts early in its infectious cycle, prior to viral DNA replication, and an additional one later, after DNA replication begins. The early primary transcripts continue to be produced after DNA replication begins. The additional primary transcript produced late in infection is large and comes from 5/6 of the 32kb adenovirus genome. This is much larger than any of the individual adenovirus mRNAs present in infected cells. Researchers found that the primary RNA transcript produced by adenovirus type 2 in the late phase was spliced in many different ways, resulting in mRNAs encoding different viral proteins. In addition, the primary transcript contained multiple polyadenylation sites, giving different 3’ ends for the processed mRNAs. In 1981, the first example of alternative splicing in a transcript from a normal, endogenous gene was characterized.〔 The gene encoding the thyroid hormone calcitonin was found to be alternatively spliced in mammalian cells. The primary transcript from this gene contains 6 exons; the calcitonin mRNA contains exons 1–4, and terminates after a polyadenylation site in exon 4. Another mRNA is produced from this pre-mRNA by skipping exon 4, and includes exons 1–3, 5, and 6. It encodes a protein known as CGRP (calcitonin gene related peptide). Examples of alternative splicing in immunoglobin gene transcripts in mammals were also observed in the early 1980s.〔 Since then, alternative splicing has been found to be ubiquitous in eukaryotes.〔 The "record-holder" for alternative splicing is a ''D. melanogaster'' gene called Dscam, which could potentially have 38,016 splice variants. 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「Alternative splicing」の詳細全文を読む スポンサード リンク
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