Pre-mRNA
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Pre-mRNA (preliminary mRNA) is a single strand of ribonucleic acid (RNA), synthesized from the DNA in the nucleus of a cell by the process transcription. It has also been called heterogenous nuclear RNA.
In eukaryotes, the pre-mRNA includes two different types of segments: exons and introns. Exons are those parts that code for polypeptides, while introns are not to be expressed and must be excised before translation may occur. This process is called splicing. Spliceosomes, small organelles found in the nucleus and made up of protein and RNA, perform the excision. They also attach new non-coding segments to the leading and lagging ends of the mRNA. These non-coding segments include a 5' cap of 7-methylguanosine and a poly-A tail through the process of polyadenylation. When the mRNA has been properly processed, it is exported out of the nucleus and to a ribosome for translation.de:Prä-mRNA pl:Pre-mRNA