Which of the following are the post-transcriptional events in an eukaryotic cell?

A. Transport of pre-mRNA to cytoplasm prior to splicing.

B. Removal of introns and joining of exons.

C. Addition of methyl group at 5’ end of hnRNA.

D. Addition of adenine residues at 3’ end of hnRNA.

E. Base pairing of two complementary RNAs.

Choose the correct answer from the options given below:

The correct option is B, C, D only.

Let us analyze each statement to determine which events are post-transcriptional modifications occurring in eukaryotic cells:

Statement A: Transport of pre-mRNA to cytoplasm prior to splicing.
This statement is incorrect because post-transcriptional processing, including splicing, occurs inside the nucleus. Only fully processed mature mRNA is transported to the cytoplasm for translation.

Statement B: Removal of introns and joining of exons.
This statement is correct. Splicing is a key post-transcriptional event in eukaryotes where non-coding sequences called introns are removed, and coding sequences called exons are joined together.

Statement C: Addition of methyl group at 5' end of hnRNA.
This statement is correct. This process is known as capping, where an unusual nucleotide, methylguanosine triphosphate, is added to the 5' end of the heterogeneous nuclear RNA (hnRNA).

Statement D: Addition of adenine residues at 3' end of hnRNA.
This statement is correct. This process is known as tailing or polyadenylation, where adenylate residues (typically 200 to 300) are added to the 3' end in a template-independent manner.

Statement E: Base pairing of two complementary RNAs.
This statement is incorrect. While base pairing can occur in specific contexts (like RNA interference or translation), it is not a standard post-transcriptional modification process required to convert hnRNA to mature mRNA in eukaryotic cells.

Therefore, statements B, C, and D are the post-transcriptional events, making "B, C, D only" the correct choice.