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5' untranslated region

3' untranslated region

5' untranslated region

Stop codon Terminator

3' untranslated region

3. A test-tube splicing system has been developed that contains all the components (snRNAs, proteins, splicing factors) necessary for the splicing of nuclear genes. When a piece of RNA containing an intron and two exons is added to the system, the intron is removed as a lariat and the exons are spliced together. If the RNA molecule added to the system has the following mutations, what intermediate products of the splicing reactions will accumulate? Explain your answer.

(a) GT at the 5' splice site is deleted.

(b) A at the branch point is deleted.

(c) AG at the 3' splice site is deleted.

(a) The GT sequence at the 5' splice site is required for the attachment of the U1 snRNP and the first cleavage reaction. If this sequence is mutated, cleavage will not take place. Thus, the original pre-mRNA with the intron will accumulate.

(b) After cleavage at the 5' splice site, the 5' end of the intron attaches to the A at the branch point in a transesterification reaction. If the A at the branch point is deleted, no lariat structure will form. The separated first exon and the intron attached to the second exon will accumulate as intermediate products.

(c) The AG sequence at the 3' splice site is required for cleavage at the 3' splice site. If this sequence is mutated, accumulated intermediate products will be: (1) the separated first exon and (2) the intron attached to the second exon, with the 5' end of the intron attached to the branch point to form a lariat structure.

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