Final answer:
Alternative poly(A) site selection involves cleaving pre-mRNA and adding a poly-A tail, which plays a crucial role in mRNA stability and export.
Step-by-step explanation:
Alternative poly(A) site selection is a process that occurs in eukaryotic cells during the maturation of pre-mRNA into mRNA. After the elongation phase of transcription, the pre-mRNA is cleaved by an endonuclease between an AAUAAA consensus sequence and a GU-rich sequence.
Following this cleavage, an enzyme known as poly-A polymerase recognizes the sequence near the 3' end of the mRNA and adds a string of approximately 200 adenine residues, termed the poly-A tail. This tail not only protects the mRNA from degradation but also signals the necessity for the mRNA to be exported to the cytoplasm for translation.
The selection of different poly(A) sites affects the ultimate mRNA produced, as different sites can lead to the creation of mRNAs with varying 3' untranslated regions (UTRs). These variations can influence mRNA stability, localization, and translational efficiency, thus playing a crucial role in gene expression regulation and the production of different protein isoforms from a single gene.
This process is also linked to the concept of alternative splicing, where a single pre-mRNA can be spliced in various ways to produce multiple mRNA variants, adding to the complexity of gene regulation and protein diversity within cells.