3' poly-A tail

Meaning: In eukaryotes, mRNA is processed in the nucleus before export. Along with the 5' cap and splicing, the 3' end is cleaved downstream of a polyadenylation signal (AAUAAA) and a poly-A tail is added by poly-A polymerase. The tail is made of adenine nucleotides added after transcription, using ATP as the substrate, with no template needed. Poly-A binding proteins attach to the tail. The tail stabilizes the mRNA against exonucleases, helps nuclear export, and promotes translation initiation. A longer tail generally means a longer mRNA lifetime in the cytoplasm, and the tail shortens over time as the mRNA is degraded.

On the MCAT: Expect questions that ask which features distinguish eukaryotic from prokaryotic mRNA (cap, tail, introns removed; prokaryotes lack these). Another common setup is mature mRNA purification: oligo-dT beads or columns bind the poly-A tail, so they isolate mRNA but not rRNA or tRNA. A passage may also mutate the AAUAAA signal or remove the tail and ask you to predict lower mRNA stability and less protein.

Common confusion: Students think the tail is coded by a run of thymines in the gene. It is not; it is added enzymatically after transcription. Also, the tail is not translated: it comes after the stop codon, in the 3' untranslated region's end, so it does not add lysines to the protein.