Before mRNA exits the nucleus, exons are removed.
The process of splicing involves cutting off certain RNA transcript segments (introns), then joining the remaining segments (exons) back together. Some genes have the ability to alternate splice, which produces several matured mRNA molecules from a single beginning transcript. Splicing, a process by which internal non-coding portions (introns) are eliminated and coding regions (exons) are combined to form functional mRNAs for exportation to the cytoplasm, is often required for maturation. The process by which the initial transcripts from a gene assumes its mature form is collectively referred to as "RNA processing." Transcripts from nuclear histone genes get a 5′ cap structure very quickly once RNA polymerase II starts to synthesize them.
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