25. During maturation process of some RNA molecules, formation of a 2′-5′ phospho-diester bond takes place. This Following statements are made about phenomenon.
A. Spliceosome mediated removal of intronic sequences occurs through the formation of a 2′- 5′ phosphodiester bond.
B. Removal of group II introns occurs through the formation of 2′-5′ phosphodiester bond.
C. Enzymatic removal of introns from the yeast  tRNA  precursors involves 2′-5′ phosphodiester bond formation.
D. RNaseP mediated 5′- end maturation of tRNA precursors involves formation of a 2′ phosphodiester bond.
Which one of the following combinations of the statements is a true representation?
(1) A only       (2) A and D
(3) A and B    (4) C and D

 

The correct combination of true statements regarding the formation of 2′-5′ phosphodiester bonds during RNA maturation is:

(3) A and B

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Explanation:

A. Spliceosome mediated removal of intronic sequences occurs through the formation of a 2′-5′ phosphodiester bond.
This is true. During spliceosome-mediated splicing of pre-mRNA, the first step involves the 2′-OH of the branch point adenosine attacking the 5′ splice site phosphate, forming a characteristic 2′-5′ phosphodiester bond in the lariat intermediate. This branched structure is essential for intron removal and exon ligation.

B. Removal of group II introns occurs through the formation of 2′-5′ phosphodiester bond.
This is also true. Group II introns self-splice via a mechanism similar to spliceosomal splicing, involving the formation of a 2′-5′ phosphodiester bond at the branch point to create a lariat intermediate.

C. Enzymatic removal of introns from the yeast tRNA precursors involves 2′-5′ phosphodiester bond formation.
This is false. Yeast tRNA introns are removed by a different enzymatic pathway involving endonucleolytic cleavage and ligation, but it does not involve a 2′-5′ phosphodiester bond or lariat formation.

D. RNase P mediated 5′-end maturation of tRNA precursors involves formation of a 2′ phosphodiester bond.
This is false. RNase P cleaves the 5′ leader sequence of pre-tRNA to generate the mature 5′ end but does not form 2′-5′ phosphodiester bonds; it is an endonuclease that cleaves phosphodiester bonds without forming branched linkages.

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Summary Table

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Conclusion

The formation of a 2′-5′ phosphodiester bond is a hallmark of the lariat intermediate in spliceosome-mediated splicing (statement A) and group II intron self-splicing (statement B). It is not involved in yeast tRNA intron removal or RNase P-mediated tRNA maturation. Therefore, the correct answer is:

(3) A and B