UNIT 4: RNA Processing

Exam Focus: The three major modifications to pre-mRNA (**Capping, Tailing, and Splicing**) are crucial. Understand the role of the **Spliceosome** and the general importance of these modifications (stability, export, translation).

Table of Contents

1. RNA processing
   • 5' cap formation
   • Polyadenylation
   • Splicing of pre-mRNA
   • Mechanism of rRNA and tRNA splicing

1. RNA processing

In eukaryotes, the primary transcript (pre-mRNA) synthesized by RNA Polymerase II must undergo several chemical modifications before it can be exported from the nucleus and translated by ribosomes.

5' cap formation

A modified guanine nucleotide, **7-methylguanosine**, is added to the 5' end of the nascent pre-mRNA molecule in a unique 5'-to-5' triphosphate linkage.

• **Function:** Essential for efficient translation initiation (ribosome binding); protects the mRNA from degradation by nucleases; and facilitates nuclear export.

Polyadenylation

The addition of a long chain of Adenine nucleotides (the **Poly-A tail**) to the 3' end of the pre-mRNA.

• **Mechanism:** The mRNA is cleaved at a specific signal sequence near the 3' end, and the Poly-A Polymerase (PAP) enzyme adds 100-250 A nucleotides without a template.
• **Function:** Increases mRNA stability and regulates the lifespan of the transcript; aids in transcription termination and translation initiation.

Splicing of pre-mRNA

The process of removing non-coding sequences (**introns**) from the pre-mRNA and ligating the coding sequences (**exons**) together to form the mature mRNA.

• **Mechanism:** Occurs within a large, complex molecular machine called the **spliceosome**, which is composed of small nuclear ribonucleoproteins (snRNPs) and numerous proteins. The process involves two transesterification reactions, often forming a **lariat structure** from the excised intron.
• **Alternative Splicing:** A mechanism where a single pre-mRNA transcript can be spliced in different ways, yielding multiple distinct mature mRNA molecules (and thus different proteins) from the same gene.

[Image of pre-mRNA splicing showing introns, exons, and the lariat structure]

Mechanism of rRNA and tRNA splicing

While protein-coding genes are spliced by the spliceosome, rRNA and tRNA precursors are often processed by different mechanisms:

• **rRNA Splicing:** Often involves self-splicing introns (ribozymes) or cleavage and modification by specific enzymes.
• **tRNA Splicing:** Pre-tRNA molecules may have short introns removed by a specialized enzyme system (not the spliceosome) involving cleavage and ligation reactions. They also undergo extensive chemical modifications to their bases.