Final answer:
The subject of the question is the control of mRNA stability through protective caps like the 5' cap and poly-A tail, and the process of mRNA degradation following translation termination at a nonsense codon.
Step-by-step explanation:
The control of RNA stability is crucial in regulating the lifespan and function of mRNA molecules within the cell. mRNA stability is influenced by various factors including the presence of protective structures, such as the 5' cap and the poly-A tail. These modifications prevent mRNA degradation by exonucleases. The 5' cap is typically a methylated guanosine triphosphate molecule, which protects mRNA from 5' exonucleases, while the poly-A tail at the 3' end helps safeguard the mRNA from 3' exonucleases. When a nonsense codon, such as UAA, UAG, or UGA, is encountered during translation, it signals the end of protein synthesis by instructing the translation machinery to release the polypeptide chain. Following the release, mechanisms are in place to degrade the mRNA, such as decapping which is performed before deadenylation, after which exonucleases like Xrn1p digest the decapped mRNA. This degradation allows nucleotides to be recycled in new transcription reactions.