Messenger RNA (Molecular Biology)

Messenger RNA (mRNA) is a fully processed RNA containing a particular open reading frame (see Gene Structure) that is translated by the ribosome into a specific polypeptide chain. In eukaryotes, all mRNAs carry a cap (1), and most are polyadenylated (2); both of these modifications can contribute to the translational efficiency of the mRNA. Normally, mRNA have the general structure from 5′ to 3′: 5′ cap and 5′ untranslated region [5'UTR], open reading frame [ORF], 3′UTR, and poly A tail. The mRNA are synthesized from the DNA sequence of the gene as a precursor, pre-mRNA, by DNA-dependent RNA polymerase II, which must be processed in order to generate the mature mRNA. This processing usually involves the removal of noncoding introns, which are specifically removed in the spliceosome (3) (see Gene Splicing), and most mRNAs are polyadenylated cotranscriptionally (2) (see Polyadenylation). Translation of the ORF initiates normally at an AUG codon, which encodes the initiating methionine residue (see Initiation Codon and Translation). Translation of the ORF is then terminated when the ribosome encounters a stop codon.