Precursor mRNA
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Precursor mRNA (pre-mRNA), also termed heterogeneous nuclear RNA (hnRNA), is an immature single strand of messenger ribonucleic acid (mRNA). pre-mRNA is synthesized from a DNA template in the cell nucleus by transcription.
Once pre-mRNA has been completely processed, it is termed "mature messenger RNA", "mature mRNA", or simply "mRNA".
[edit] 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, while 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 the pre-mRNA. These include a 5' cap of 7-methylguanosine and a poly-A tail.
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.
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