In genetics, the 3' UTR (read as 3 prime untranslated region) is a particular section of messenger RNA (mRNA). Genetics (from the Greek genno γεννώ= give birth) is the science of genes, heredity, and the variation of organisms. ... The interaction of mRNA in a eukaryote cell. ...

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 (the process is called 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 (three nucleic acids which tell the enzyme to stop). Anything beyond this stop codon is part of the 3 prime untranslated region (3' UTR). RNA codons. ...

In a diagram:

 start stop codon codon 

 ---------|-------------------|--------- 5'-UTR translated RNA 3'-UTR 

Many functional elements occur in the 3' UTR:

• Polyadenylation signals (usually AAUAAA, or a slight variant) that mediate the cleavage of the transcript ~30bp 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.
• SECIS elements, which can occur in the 3' UTRs of eukaryotic mRNAs, direct the ribosome to translate UGA 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.