Final answer:
It is true that the 3' poly (A) tail consists of a long sequence of adenine nucleotides added to the 3' end of pre-mRNA, which is crucial for mRNA protection and export from the nucleus.
Step-by-step explanation:
The statement that the 3' poly (A) tail is a long sequence of several hundreds of adenine nucleotides added to the 3' end of the pre-mRNA is true. The poly-A tail, typically around 200 adenine residues long, is a crucial modification to the pre-mRNA, as it serves to protect the mRNA from enzymatic degradation and assists in the export of mRNA from the nucleus into the cytoplasm. This tail is added after transcription is complete, when an enzyme called poly-A polymerase recognizes a specific sequence near the 3' end of the newly synthesized mRNA and adds the adenine string.