29. In type II splicing
(1) a ‘G-OH’ from outside makes a nucleophilic attack on 5′-P of first base of intron
(2) a free 2’-OH of an internal adenosine makes a nucleophilic attack on 5′-P of first base of intron
(3) A 3′-OH of an internal adenosine makes a nucleophilic attack on 5′-P of first base of intron
(4) the hydrolysis of last base of exon is carried out by U2/U4/U6
The correct answer is:
(1) a ‘G-OH’ from outside makes a nucleophilic attack on 5′-P of first base of intron
Explanation:
Type II splicing refers to the splicing mechanism of group I introns, which is distinct from the spliceosomal (type I) and group II intron splicing.
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In group I intron splicing (type II splicing), the first step involves a free guanosine (G) nucleotide or guanosine cofactor (G-OH) from outside the intron acting as a nucleophile.
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This free G-OH attacks the 5′ splice site phosphate (5′-P) at the first base of the intron, cleaving the exon-intron junction.
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This is different from spliceosomal and group II intron splicing, where a 2′-OH of an internal adenosine within the intron makes the nucleophilic attack, forming a lariat intermediate.
Why other options are incorrect:
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(2) a free 2′-OH of an internal adenosine makes a nucleophilic attack on 5′-P of first base of intron
This describes group II intron splicing and spliceosomal splicing (type I splicing), not type II splicing. -
(3) A 3′-OH of an internal adenosine makes a nucleophilic attack on 5′-P of first base of intron
This is incorrect; the nucleophile is the 2′-OH, not the 3′-OH. -
(4) the hydrolysis of last base of exon is carried out by U2/U4/U6
This is incorrect and unrelated to the nucleophilic attack mechanism in type II splicing. U2, U4, and U6 are spliceosomal snRNAs involved in spliceosomal splicing, not group I intron splicing.
Summary:
| Option | Description | Correctness |
|---|---|---|
| (1) | Free G-OH attacks 5′-P of intron (group I intron) | Correct for type II splicing |
| (2) | 2′-OH of internal adenosine attacks 5′-P (group II) | Incorrect for type II splicing |
| (3) | 3′-OH of internal adenosine attacks 5′-P | Incorrect |
| (4) | Hydrolysis by U2/U4/U6 snRNAs | Incorrect |
Additional Notes:
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Group I introns (type II splicing) use an exogenous guanosine nucleophile to initiate splicing.
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Group II introns and spliceosomal introns use an internal branch point adenosine’s 2′-OH for the nucleophilic attack, forming a lariat intermediate.
6 Comments
Lokesh kumawat
June 7, 2025Done
Prami Masih
June 8, 2025👍👍👍
Prami Masih
June 8, 2025👍👍👍👍
Santosh Saini
November 3, 2025Option 1st is correct bcz a ‘G-OH’ from outside makes a nucleophilic attack on 5′-P of first base of intron
Pooja
November 4, 2025a ‘G-OH’ from outside makes a nucleophilic attack on 5′-P of first base of intron
Heena Mahlawat
November 5, 2025Option 1