What is added to the 3' end of eukaryotic mRNA during processing?

• A. A poly-A tail of about 50-250 adenines added
• B. A cap at the 3' end
• C. A poly-C tail
• D. A poly-U tail

Answer: (A)

Post-transcriptional processing in eukaryotes includes adding a protective poly-A tail to the 3' end of the mRNA. After transcription, the pre-mRNA is cleaved at a specific polyadenylation signal, and about 50 to 250 adenine residues are added by poly(A) polymerase. This poly-A tail helps stabilize the transcript by shielding it from exonucleases, promotes export from the nucleus to the cytoplasm, and enhances translation efficiency by enabling binding of poly(A)-binding proteins. The cap, added to the 5' end, is a separate modification and not placed at the 3' end. Tails made of cytosine or uracil are not characteristic features of this 3' end processing step.