U01.01.014 Splicing of pre-mRNA

Splicing is a fundamental process in eukaryotic gene expression that transforms precursor messenger RNA (pre-mRNA) into mature mRNA. This step ensures only the coding regions (exons) are included in the final mRNA, which is then used for protein synthesis. Here’s a detailed breakdown for Step 1 of the USMLE:

The Precursor: Pre-mRNA

Pre-mRNA is the initial transcript produced by RNA polymerase II in the nucleus.
It contains both coding sequences (exons) that will be translated into protein and non-coding sequences (introns) that are removed during splicing.
The number of introns and exons can vary greatly between genes.

The Splicing Machinery: Small Nuclear Ribonucleoproteins (snRNPs)

SnRNPs are protein-RNA complexes essential for splicing.
They consist of a small nuclear RNA (snRNA) molecule associated with various proteins.
Different snRNPs recognize specific splice sites within the pre-mRNA:
- 5′ splice site (donor site): Located at the 5′ end of the intron.
- 3′ splice site (acceptor site): Located at the 3′ end of the intron.
- Branch point: A consensus sequence within the intron.

Table 1: Key Spliceosome Components and Functions

Component	Function
U1 snRNP	Recognizes the 5′ splice site
U2 snRNP	Binds to the branch point and facilitates intron looping
U4/U5/U6 snRNP complex (Spliceosome)	Catalyzes the cleavage at splice sites and exon ligation

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The Splicing Process: A Multi-Step Orchestration

Splicing involves a series of coordinated steps mediated by snRNPs and other splicing factors:

Assembly: SnRNPs and other splicing factors assemble on the pre-mRNA, recognizing splice sites and the branch point.
Cleavage: The 2′ hydroxyl group of an Adenine nucleotide at the branch point attacks the phosphodiester bond at the 3′ splice site, cleaving the intron from the pre-mRNA. The intron remains capped by a lariat structure.
Exon Ligation: The 5′ splice site of the upstream exon attacks the phosphodiester bond at the 5′ end of the intron (lariat), joining the exons together. The intron is released as a lariat-shaped molecule and is later degraded.
Spliceosome Disassembly: SnRNPs and other factors dissociate from the mature mRNA.

Table 2: Splicing Steps and Key Events

Step	Key Event
Assembly	SnRNPs and splicing factors recognize splice sites and branch point
Cleavage	Intron is cleaved from pre-mRNA at 3′ splice site, forming a lariat structure
Exon Ligation	Exons are joined together by phosphodiester bond formation
Disassembly	Spliceosome components dissociate from mature mRNA

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Clinical Significance of Splicing Abnormalities

Mutations that disrupt snRNP assembly or splice site recognition can lead to various diseases. Here’s an example:

Spinal Muscular Atrophy (SMA): Mutations in the SMN protein, which is crucial for snRNP assembly, lead to abnormal splicing of pre-mRNA for genes essential for motor neuron function. This results in motor neuron degeneration, muscle weakness, and hypotonia.

Key Takeaways for Step 1:

Splicing is a critical step in eukaryotic gene expression, ensuring only coding regions are included in mature mRNA.
SnRNPs play a vital role in recognizing splice sites and facilitating intron removal.
Mutations affecting splicing can have severe consequences, contributing to various diseases.

USMLE Step 1 Review Series

Curriculum

U01.01.014 Splicing of pre-mRNA

The Precursor: Pre-mRNA

The Splicing Machinery: Small Nuclear Ribonucleoproteins (snRNPs)

The Splicing Process: A Multi-Step Orchestration

Clinical Significance of Splicing Abnormalities

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