In eukaryotic post-transcriptional processing, what modification is added to the 5' end of the newly transcribed pre-mRNA?

Answer and explanation

Correct answer: D. 7-methylguanosine cap

  1. Step 1: Nascent eukaryotic pre-mRNA transcripts undergo multiple enzymatic processing steps in the nucleus prior to cytoplasmic export.
  2. Step 2: Shortly after transcription begins, an unusual methylated guanosine nucleotide, 7-methylguanosine (), is covalently linked to the terminal nucleotide via an atypical triphosphate bridge.
  3. Step 3: This cap prevents degradation by cellular exonucleases and facilitates ribosome recognition and binding during translation initiation.
Answer
7-methylguanosine cap.
Why not the others: Poly-A tail consists of 200–300 adenylate residues added post-transcriptionally to the 3-prime end (not 5-prime) of the transcript; Intron spliceosome is the macromolecular ribonucleoprotein complex that excises introns rather than an end-capping moiety; Ribosomal RNA subunit associates with processed mature mRNA only during translation in the cytoplasm.

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