Eukaryotic mRNA 3′ end formation: where are most protein-coding transcripts cleaved relative to the polyadenylation signal? Select the correct statement.

Introduction / Context:Most eukaryotic mRNAs undergo co-transcriptional 3′ end processing, including endonucleolytic cleavage and addition of a poly(A) tail. The location of cleavage relative to the AAUAAA polyadenylation signal is a core principle in RNA processing.

Given Data / Assumptions:

• Canonical polyadenylation signal: AAUAAA (with variants).
• Cleavage typically occurs 10–30 nucleotides downstream of this signal.
• Poly(A) polymerase adds a poly(A) tail after cleavage.

Concept / Approach:Cleavage and polyadenylation specificity factor (CPSF), cleavage stimulation factor (CstF), and other factors recognize the signal and downstream elements, specifying cleavage after the signal in the 3′ untranslated region.

Step-by-Step Solution:Locate AAUAAA in pre-mRNA.Identify the cleavage site 10–30 nt downstream (toward the 3′ end) of the signal.Poly(A) is then added to the newly formed 3′ hydroxyl.

Verification / Alternative check:Experimental mapping of 3′ ends by 3′ RACE and global studies confirm cleavage downstream of the signal, not before it and not at the 5′ end.

Why Other Options Are Wrong:Cleavage occurs after the signal downstream, not before. The 5′ end language is irrelevant to polyadenylation. Sites are not random; they depend on specific sequence and protein factor recognition.

Common Pitfalls:Confusing the AAUAAA position with the exact cleavage nucleotide; the cut is typically downstream by a defined distance with auxiliary elements (for example, U/GU-rich regions).

Final Answer:At the 3′ end just after a polyadenylation signal.