poly-A tail
/ PAH-lee-AY TAYL /
If the 5-prime cap is a seal on the front of a mailed message, the poly-A tail is a protective streamer tied to the back. It is a long string of the same RNA letter — adenine, A — added to the tail end (the 3-prime end) of a messenger RNA, often a hundred or two hundred A's in a row. No other letters, just A after A after A.
The tail is not copied from a gene; it is added afterward by a dedicated enzyme during a step called polyadenylation. Near the end of a transcript there is a signal sequence; once the cell spots it, it cuts the RNA there and an enzyme called poly-A polymerase tacks on the long run of A's. Special proteins then coat this tail, and that protein coat is what actually does the protecting and the signaling — the A's are really a binding platform.
The poly-A tail matters because it largely controls how long a message survives and how readily it gets translated. A long, well-protected tail means a stable, frequently read message; as the tail is gradually nibbled shorter over time, the mRNA becomes a candidate for destruction. In this way the tail acts like a slow-burning fuse or an expiry timer, letting the cell decide not just whether to make a message but how long to keep using it.
In a frog egg, certain mRNAs sit waiting with very short tails, essentially switched off. Only after fertilization does the cell lengthen their poly-A tails, abruptly switching those messages on so the embryo can start making the proteins it suddenly needs.
Lengthening a poly-A tail can switch a stored mRNA on, as happens right after a frog egg is fertilized.
The poly-A tail is added after transcription and is not encoded in the gene; it sits at the 3' end, opposite the 5' cap. Most histone mRNAs are a famous exception and lack a poly-A tail.