27. Following statements were made about the post-transcriptional processing of RNA in eukaryotes.
A. Soon after transcription initiation, RNA polymerase II pauses -30 nucleotides downstream from the site of initiation until the Cap structure is added to the 5′ end of the nascent pre- mRNA
B. The 5′ splice sites are functionally divergent whereas the 3′ sites are functionally equivalent
C. In addition to helping in recognition of the splice sites, the exon definition also functions as a splicing regulator by allowing pairing and linking of adjacent 5’ and 3′ splice sites.
D. The intron definition mechanism applies only to the larger introns (above 500 nucleotides length) and assists in achieving alternate splicing.
E. The splicing reactions carried out in vitro have the first and second trans- revealed that esterification reactions are reversible.
Which one of the following combination of statements correct?
(1) A, B and D      (2) B, C and D
(3) B, D and E      (4) A, C and E

The correct combination of true statements about post-transcriptional processing of RNA in eukaryotes is:

(4) A, C and E

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Explanation of Each Statement:

A. Soon after transcription initiation, RNA polymerase II pauses ~30 nucleotides downstream from the site of initiation until the Cap structure is added to the 5′ end of the nascent pre-mRNA
This is true. RNA polymerase II undergoes promoter-proximal pausing shortly after initiation, typically around 20–60 nucleotides downstream. During this pause, the 5′ cap structure (7-methylguanosine cap) is rapidly added to the nascent transcript, which is crucial for RNA stability and further processing.

B. The 5′ splice sites are functionally divergent whereas the 3′ sites are functionally equivalent
This is false. Both 5′ and 3′ splice sites show sequence variability, but 5′ splice sites tend to be more conserved and functionally critical for spliceosome recognition. The 3′ splice sites are also variable and not functionally equivalent. Thus, the statement oversimplifies and is inaccurate.

C. In addition to helping in recognition of the splice sites, the exon definition also functions as a splicing regulator by allowing pairing and linking of adjacent 5′ and 3′ splice sites
This is true. The exon definition model posits that splicing factors recognize exons as units by bridging adjacent splice sites, facilitating accurate splice site pairing and regulation of alternative splicing.

D. The intron definition mechanism applies only to the larger introns (above 500 nucleotides length) and assists in achieving alternate splicing
This is false. The intron definition mechanism primarily applies to short introns, where the spliceosome recognizes the intron boundaries directly. Exon definition is more relevant for larger introns. Thus, the statement reverses the known roles of intron and exon definition.

E. The splicing reactions carried out in vitro have revealed that the first and second trans-esterification reactions are reversible
This is true. Biochemical studies of splicing in vitro have shown that the two transesterification steps (first forming the lariat intermediate, second ligating exons) are reversible under certain conditions.

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Summary Table

Conclusion

The correct combination of true statements is A, C, and E, corresponding to option (4).