Precursor mRNA
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Precursor mRNA, more commonly termed pre-mRNA, is an incompletely processed single strand of messenger ribonucleic acid (mRNA), synthesized from a DNA template in the nucleus of a cell by transcription. It has also been called heterogeneous nuclear RNA (or hnRNA) and primary transcript. Once pre-mRNA has been completely processed, it is termed "mature messenger RNA", "mature mRNA", 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.
Pre-mRNA is sometimes incorrectly referred to as "preliminary mRNA".
Pre-mRNA is also commonly known as "primary RNA transcript".
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Transcription - RNA splicing (Precursor mRNA, Intron/Exon, snRNP, Spliceosome, Alternative splicing) - Polyadenylation - 5' cap
