What is mRNA capping and polyadenylation and why are these modifications important?

• A. They stabilize the mRNA in the nucleus but have no role in export.
• B. They are not essential for translation.
• C. They protect mRNA from degradation and aid export and translation.
• D. They occur only in bacterial mRNA.

Answer: (C)

mRNA capping and polyadenylation are end-modification steps that happen to mature eukaryotic mRNA. A 5' cap is added early in transcription, and a poly(A) tail is appended to the 3' end after transcription. These features protect the message from degradation by nucleases, help the mRNA be exported from the nucleus, and enable efficient translation in the cytoplasm. The cap is recognized by cap-binding proteins that recruit the translation machinery, while the poly(A) tail binds poly(A)-binding proteins that interact with initiation factors to promote ribosome recruitment and mRNA stability. Because of these roles, these modifications are essential for productive gene expression. The other statements misstate their functions or occurrence: the cap and tail do more than just stabilize in the nucleus, they actively support export and translation; they are essential for translation; and they are characteristic of eukaryotic mRNA rather than bacterial mRNA.