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''See main article Primary transcript for more details'' Precursor mRNA (pre-mRNA) is an immature single strand of messenger ribonucleic acid (mRNA). Pre-mRNA is synthesized from a DNA template in the cell nucleus by transcription. Pre-mRNA comprises the bulk of heterogeneous nuclear RNA (hnRNA). The term hnRNA is often used as a synonym for pre-mRNA, although, in the strict sense, hnRNA may include nuclear RNA transcripts that do not end up as cytoplasmic mRNA. Once pre-mRNA has been completely processed, it is termed "mature messenger RNA", "mature mRNA", or simply "mRNA". ==Processing== Eukaryotic pre-mRNA exists only briefly before it is fully processed into mRNA. Pre-mRNAs include two different types of segments, exons and introns. Exons are segments that are retained in the final mRNA, whereas introns are removed in a process called splicing, which is performed by the spliceosome (except for self-splicing introns). Additional processing steps attach modifications to the 5' and 3' ends of Eukaryotic pre-mRNA. These include a 5' cap of 7-methylguanosine and a poly-A tail. In addition, eukaryotic pre-mRNAs have their introns spliced out by spliceosomes made up of small nuclear ribonucleoproteins.〔Weaver, Robert F. (2005). ''Molecular Biology'', p.432-448. McGraw-Hill, New York, NY. ISBN 0-07-284611-9.〕 When a pre-mRNA strand has been properly processed to an mRNA sequence, it is exported out of the nucleus and eventually translated into a protein – a process accomplished in conjunction with ribosomes. 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「Precursor mRNA」の詳細全文を読む スポンサード リンク
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