What is a poly-A tail?

A poly-A tail refers specifically to a long sequence of adenine nucleotides that is added to the 3' end of an mRNA molecule during the post-transcriptional modification process. This addition of the poly-A tail serves several important functions in the stability and regulation of the mRNA. It enhances the stability of the mRNA by protecting it from degradation by exonucleases, aids in the export of the mRNA from the nucleus to the cytoplasm, and plays a crucial role in the initiation of translation.

The process of adding the poly-A tail is referred to as polyadenylation, which typically occurs after the transcription of a gene has been completed. The presence of the poly-A tail is important for the proper functioning of eukaryotic mRNA, as it influences how efficiently the mRNA is translated into protein.

The other options refer to different aspects of molecular biology. The sequence of amino acids at the beginning of a protein pertains to the translation process, while modifications during transcription usually involve the capping of the 5' end of mRNA or splicing out introns.