DNA is first transcribed into pre-mRNA in the nucleus. This pre-mRNA is larger than mRNA, because pre-mRNA contains introns as well as exons. After transcription, RNA splicing occurs in which introns are removed and exons are joined together by which the size of RNA decreases.After that , at the 5' end of pre-mRNA, 5' capping occurs and at the 3' end of pre-mRNA, 3' polyadenylation occurs. This finally mature RNA is called mRNA.
