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Post-Transcriptional RNA Processing

Problems

The Chemistry of the Primary RNA Transcript Splicing Reaction and Alternative Splicing

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Problem : The splicing reaction results in the formation of two structures. Starting with a structure composed of two exons (exon 1 and exon 2) that are separated by an intron, what will be the composition (intron, exon 1, exon 2) or the two splicing products?

One splicing product will be the intron; the other product will be exon 1 and exon 2 joined together.

Problem : Describe the event that takes place during the first splicing step.

In the first step of the splicing reaction, the 2' –OH of the branch point A of the intron attacks the phosphoryl group of the G in the 5' splice site.

Problem : What is the name of the looped structure that is formed after the first splicing step?

The lariat structure.

Problem : Describe the event that occurs in the second splicing step.

In the second splicing step, the newly formed 3' –OH if exon 1 attacks the phosphoryl group of the nucleotide that is at the 5'-most end of exon 2.

Problem : What is the benefit of alternative splicing? Could there also be drawbacks to alternative splicing?

The main benefit of alternative splicing is increased diversity of the protein products possible from one segment of DNA. While this can be a benefit, it can also be detrimental to a cell because alternative splicing could lead to the deletion of an essential exon segment that could lead to problems as severe as cell death. Such mutations do not often occur and as a result, alternative splicing is more beneficial than it is costly.

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