In genetics, the three prime untranslated region (3' UTR) is a particular section of messenger RNA (mRNA).

Like all strands of nucleic acid, mRNA is directional. One end is the 5' (five prime) end, the other is the 3' (three prime) end.

When a ribosome uses messenger RNA's sequence to build a protein through translation, it starts reading from the 5' end, until it encounters a start codon. Amino acids are then added to the protein until the enzyme encounters a stop codon, which are three nucleic acids which tell the enzyme to stop. Anything beyond this stop codon is part of the three prime untranslated region.

In a diagram:

start stop codon codon

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- 5'-UTR Coding Region 3'-UTR

Many regulatory sequences can be found in the 3' UTR:

• Polyadenylation signals, usually in the form of AAUAAA, or a slight variant that mediate the cleavage of the transcript approximately 30 base pairs downstream of the signal, followed by the addition of several hundred adenine residues (poly-A). The poly-A tail is thought to protect the mRNA from degradation.
• Binding sites for miRNAs, which play a role in gene regulation
• SECIS elements, which can occur in the 3' UTRs of eukaryotic mRNAs, direct the ribosome to translate some UGA stop codons as selenocysteines.
• The histone downstream element is analogous to polyadenylation in function, but has different sequence characteristics, and is used only for histone genes.

RNAGene expression

3' UTR