Q.27

The 5′ cap of eukaryotic mRNAs contains

(A) a modified guanine nucleotide

(B) a modified adenine nucleotide

(C) a modified cytosine nucleotide

(D) a modified uracil nucleotide

5′ Cap of Eukaryotic mRNA: Structure and MCQ Answer

The 5′ cap of eukaryotic mRNAs is a critical modification consisting of a modified guanine nucleotide added to the 5′ end during post-transcriptional processing. This structure enhances mRNA stability and translation efficiency. In the given multiple-choice question, option (A) is correct.

Correct Answer

Option (A) a modified guanine nucleotide accurately describes the 5′ cap. Specifically, it is 7-methylguanosine (m7G) linked via a 5′-5′ triphosphate bridge to the first nucleotide of the mRNA. This cap-0 structure forms co-transcriptionally and protects against exonucleases while aiding ribosome binding.

Option Explanations

• (A) Modified Guanine Nucleotide: Correct, as the cap features N7-methylguanosine, often with additional ribose methylations in cap-1 or cap-2 forms, enabling eIF4E recognition for translation initiation.

• (B) Modified Adenine Nucleotide: Incorrect; adenine modifications occur in poly-A tails at the 3′ end, not the 5′ cap, which is guanine-specific in eukaryotes.

• (C) Modified Cytosine Nucleotide: Incorrect; cytosine plays no role in standard 5′ capping, which exclusively uses guanosine for mRNA maturation.

• (D) Modified Uracil Nucleotide: Incorrect; uracil is absent in mRNA (replaced by thymine in DNA), and the cap does not involve pyrimidine modifications like uracil.