Question

What is the minimum number of transesterification reactions needed to splice an intron from a pre-mRNA transcript? Explain.

Short answer

Two transesterification reactions are needed to splice an intron from pre-mRNA.

Step-by-step solution

Understanding Transesterification

Transesterification is a reaction where a hydroxyl group attacks a phosphodiester bond, causing cleavage and reformation of the bond. In splicing, these reactions rearrange the RNA to remove introns.

Identifying Splicing Process

In the process of splicing, introns are removed from an RNA sequence, and exons are joined together. This typically involves two main transesterification reactions.

First Transesterification Reaction

The first transesterification occurs when the 2'-hydroxyl group of the adenosine within the branch site attacks the 5' splice site at the exon-intron junction, creating a lariat structure with the intron.

Second Transesterification Reaction

The second transesterification involves the 3'-hydroxyl of the upstream exon attacking the 3' splice site, which leads to the excision of the intron in a lariat form and joining of the exons.

Key concepts

Transesterification

Transesterification is a fascinating chemical reaction pivotal to RNA splicing. It involves a hydroxyl group performing a nucleophilic attack on a phosphodiester bond. This attack is all about rearranging chemical bonds, which is crucial for RNA modification.
In simpler terms, it helps to stitch together the exons, the coding sections of RNA, by cleaving and forming new bonds.

In RNA splicing, there are two key transesterification reactions:

• The first one sets the stage by forming a loop-like structure called a lariat with the intron.
• The second reaction ultimately cuts out the intron and stitches the exons back together.

These steps are essential for creating a functional messenger RNA (mRNA) ready to be translated into proteins.

pre-mRNA processing

The journey of pre-mRNA to becoming mature mRNA is a crucial part of gene expression. Pre-mRNA is the raw transcript directly synthesized from DNA. It includes both introns and exons.
This transcript initially contains sequences that need to be removed and processed; thus, it undergoes significant modifications, known as pre-mRNA processing.

This processing involves:

• Capping: Adding a 7-methylguanylate cap at the 5’ end to stabilize the mRNA.
• Polyadenylation: Adding a poly-A tail at the 3’ end, which protects the mRNA from degradation.
• Splicing: Removing introns and joining exons to produce a continuous coding sequence.

This meticulous processing ensures that only the necessary coding portions are translated into proteins.

Intron Removal

The removal of introns from RNA is a critical step in creating a functional mRNA molecule. Introns are non-coding regions in the RNA transcript that need to be excised before the RNA can be translated into a protein.

During splicing, introns are removed in a two-step process involving transesterification reactions:

• Lariat Formation: The first transesterification reaction forms a lariat structure, effectively looping out the intron.
• Exon Joining: The second reaction cuts out the intron and joins the exons, leaving a linear RNA ready for translation.

These precise reactions ensure the RNA is error-free and retains only the necessary coding sequences for protein synthesis.