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Circular RNA (or circRNA) is a type of RNA which, unlike the better known linear RNA, forms a covalently closed continuous loop, i.e., in circular RNA the 3' and 5' ends normally present in an RNA molecule have been joined together. This feature confers numerous properties to circular RNAs, many of which have only recently been identified. Many circular RNAs arise from otherwise protein-coding genes, but circular RNAs produced in the cell have not been shown to code for proteins. They have therefore been categorized as noncoding RNA. Some circular RNAs have recently shown potential as gene regulators. Like many other alternative noncoding isoforms, the biological function of most circular RNAs are unclear. Because circular RNAs do not have 5' or 3' ends, they are resistant to exonuclease-mediated degradation and are presumably more stable than most linear RNAs in cells. == RNA splicing == In contrast to genes in bacteria, eukaryotic genes are split by non-coding sequences known as introns. In eukaryotes, as a gene is transcribed from DNA into a messenger RNA (mRNA) transcript, intervening introns are removed, leaving only exons in the mature mRNA, which can subsequently be translated to produce the protein product. The spliceosome,〔 a protein-RNA complex located in the nucleus, catalyzes splicing in the following manner: # The spliceosome recognizes an intron, which is flanked by specific sequences at its 5’ and 3’ ends, known as a donor splice site (or 5’ splice site) and an acceptor splice site (or 3’ splice site), respectively. # The 5’ splice site sequence is then subjected to a nucleophilic attack by a downstream sequence called the branch point, resulting in a circular structure called a lariat. # The free 5’ exon then attacks the 3’ splice site, joining the two exons and releasing the intron lariat. The intron lariat is subsequently debranched and quickly degraded.〔 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「Circular RNA」の詳細全文を読む スポンサード リンク
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