All mRNAs contains two regions. A coding region which consists of a series of triplet codons representing the amino acid sequence of the coded protein, starting with and AUG (initiation codon) and ending with a termination codon. This is referred to as the open reading frame (ORF). Extra regions are present at both the 5′ and 3′ ends of an mRNA. The sequence at the 5′ end, preceding the initiation codon, is the leader sequence. The sequence following the termination codon, at the 3′ end, is called a trailer. They are also referred to as 5′ and 3′ untranslated regions, respectively. The UTR sequences are part of the transcription unit, but these are not used to code for the protein.
Eukaryotic mRNA constitutes only a small proportion of the total cellular RNA (approx. 3%). The half-life of mRNA in the yeast is short, ranging from 1-60 minutes. The mRNA is highly stable in higher eukaryotes: animal cell mRNAs have half-lives ranging from 4-24 hours.