Alternative splicing can process the same pre-mRNA into different forms of mRNA.

Question

Alternative splicing can process the same pre-mRNA into different forms of mRNA. Which one of the following reasons best explains why this pre-mRNA can't be processed into a mRNA containing only (Exon 1-Intron A-Exon 3)?

The spliceosome can not excise RNA sequences bounded by two 5' splice sites.

A. The spliceosome can not excise RNA sequences bounded by an upstream 5' splice site and a downstream 3' splice site. B.

The spliceosome can not excise RNA sequences bounded by two 5' splice sites.

C. The spliceosome can not excise RNA sequences bounded by two 3' splice sites. D. The spliceosome can not excise RNA sequences bounded by an upstream 3' splice site and a downstream 5' splice site.

Explanation / Answer

Splice sites are located at the two ends of an intron- a 5' splice site at the start of the intron ( a 5' GU) and a 3' splice site at the end of the intron ( a 3' AG). These sequences are conserved & are recognised by spliceosomes during splicing the intron out of the pre-mRNA.

Here, the sequence of the pre-mRNA can be written as-

exon1---- 5' GU- intron A- 3'AG------- exon 2------- 5' GU- intron B- 3'AG--------- exon 3

Now if exon1- intron A- exon 3 is to be formed; then a 5' GU is to be there in front of exon 2 & a 3' AG at the end of intron B. Although the latter is present, the 5' splice site in front of exon 2 is absent; hence no splicing in this way.

The correct option is

(C)- The spliceosome can not excise RNA sequences bounded by two 3' splice sites.

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