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Precursor mRNA, more commonly termed pre-mRNA, is an incompletely processed single strand of ribonucleic acid (RNA), synthesized from a DNA template in the nucleus of a cell by transcription. It has also been called heterogeneous nuclear RNA or hnRNA or a primary transcript. Once pre-mRNA has been completely processed, it is termed mature mRNA, mature messenger RNA, or simply mRNA.
Eukaryotic pre-mRNA exists only briefly before it is fully processed into mature mRNA. Pre-mRNA includes two different types of segments, exons and introns. Most of exons encode protein, while introns do not and must be excised before translation. This process is called splicing. Spliceosomes, small organelles found in the nucleus and composed of protein and RNA, perform the excision. Additional processing steps attach modifications to the "front" (5') and "back" (3') ends of the mRNA. These non-coding segments include a 5' cap of 7-methylguanosine and a poly-A tail. When the mRNA has been properly processed, it is exported out of the nucleus and to ribosomes for translation.
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