explain that, in eukaryotes, the RNA molecule formed following transcription (primary transcript) is modified by the removal of non-coding sequences (introns) and the joining together of coding sequences (exons) to form mRNA

Protein Synthesis – RNA Processing in Eukaryotes

Learning Objective

Explain that, in eukaryotes, the RNA molecule formed following transcription (primary transcript) is modified by the removal of non‑coding sequences (introns) and the joining together of coding sequences (exons) to form mature messenger RNA (mRNA).

Key Concepts

• Primary transcript (pre‑mRNA): the initial RNA copy of a gene produced by RNA polymerase II.
• Introns: non‑coding regions that are removed during RNA processing.
• Exons: coding regions that remain in the final mRNA.
• Splicing: the enzymatic process that removes introns and ligates exons.
• 5′ capping and 3′ poly‑A tail: additional modifications that protect mRNA and aid translation.

Steps of RNA Processing

1. 5′ Capping: A modified guanine nucleotide is added to the 5′ end of the nascent transcript.
2. Splicing

   • The spliceosome recognises conserved sequences at the intron‑exon boundaries (5′ splice site, branch point, 3′ splice site).
   • Introns are excised as a lariat structure.
   • Exons are ligated together to produce a continuous coding sequence.

3. 3′ Poly‑A Tail Addition: A string of adenine nucleotides (≈200 A residues) is added to the 3′ end.

Comparison of Primary Transcript and Mature mRNA

Mechanism of Splicing – The Spliceosome

The spliceosome is a large ribonucleoprotein complex composed of small nuclear RNAs (snRNAs) and associated proteins (snRNPs). It carries out two transesterification reactions:

\$\$

\begin{aligned}

\text{First step:}&\quad \text{5′ splice site attacks the branch point adenosine, forming a lariat.}\\

\text{Second step:}&\quad \text{3′ splice site attacks the 5′ exon, joining exons together.}

\end{aligned}

\$\$

Regulation of Alternative Splicing

Alternative splicing allows a single gene to produce multiple protein isoforms. The choice of which introns are removed can be influenced by:

• Splicing enhancers and silencers within the pre‑mRNA.
• RNA‑binding proteins (e.g., SR proteins, hnRNPs).
• Cell‑type specific expression patterns.

Biological Significance

RNA processing ensures that only the coding information is retained in the mRNA, protects the transcript from degradation, and provides additional layers of gene regulation through alternative splicing.

Quick Revision Checklist

• Identify the three main modifications of a primary transcript.
• Describe the role of the spliceosome and the two transesterification steps.
• Explain how alternative splicing contributes to protein diversity.