Week 32 Lecture 10 mRNA Stability_New

Lecture Overview

This lecture, presented by Dr. Jerome Korzelius at the University of Kent, focuses on mRNA stability as part of the Gene Expression and its Control course (BIOS5010). The lecture outlines essential concepts in molecular biology, specifically the lifecycle of mRNA, its processing, degradation, and mechanisms affecting stability.

1. Gene Expression: The Basics

  • Gene expression progresses from DNA to RNA to protein through several steps:

    • Replication: Duplication of DNA molecules.

    • Transcription: Synthesis of mRNA from DNA templates.

    • Translation: Production of proteins based on mRNA sequence.

  • This process showcases the central dogma of molecular biology, highlighting the flow of genetic information.

2. mRNA Characteristics

  • Structure: mRNA is single-stranded and highly processed before functioning in protein synthesis.

  • Processing Needs:

    • mRNA synthesized by RNA Polymerase II requires modifications such as splicing, capping, and transport for translation into proteins.

    • Unprocessed pre-mRNA remains within the nucleus until it undergoes complete processing.

  • Length and Stability: mRNA is inherently unstable; thus, stabilizing modifications are crucial for its function.

3. Differences Between pre-mRNA and Mature mRNA

  • Pre-mRNA: Found only in the nucleus, unexported until processing is fully completed.

  • Mature mRNA: Contains joined exons, a 5' cap, and a 3' poly-A tail, essential for translation by ribosomes in the cytoplasm.

4. mRNA Processing Steps in Eukaryotes

  • Five Main Stages:

    1. 5' Capping: Addition of a modified guanine nucleotide to the 5' end.

    2. RNA Splicing: Introns are excised, and exons are connected to form a continuous sequence.

    3. 3' Polyadenylation: Addition of a tail of adenine nucleotides which enhances stability.

5. The Functional Importance of UTRs

  • Untranslated Regions (UTRs): Present at both ends of mRNA (5' and 3' UTRs), which are crucial for mRNA stability.

  • Influence on mRNA: UTRs and the secondary structure of mRNA affect its stability, degradation, translation efficiency, and localization within the cell.

6. mRNA Instability and Degradation Mechanisms

  • Ribonucleases Role: mRNA degradation is primarily driven by ribonucleases:

    • Endoribonucleases: Cleave RNA at internal sites.

    • Exoribonucleases: Remove nucleotides from the ends of RNA molecules.

  • Prokaryotic mRNA Degradation:

    • For example, in E. coli, mRNA is degraded via endonuclease and exonuclease activities, resulting in action from RNase E, which disrupts mRNA functionality.

7. mRNA Degradation in Eukaryotes

  • Deadenylation: Most eukaryotic mRNA degradation begins with the removal of the poly-A tail from the 3' end and is followed by decapping at the 5' end.

  • Two Pathways of mRNA decay:

    • Pathway 1 (5’ to 3’ degradation): Triggered by decapping of the mRNA allowing fast decay by exonucleases.

    • Pathway 2 (3’ to 5’ degradation): Involves exonuclease complexes like the Exosome which degrade mRNA from the 3' end.

  • Quality Control: The TRAMP complex identifies and targets improperly processed mRNA for degradation.

8. Nonsense-Mediated Decay (NMD)

  • Function: NMD selectively degrades mRNAs containing premature termination codons (PTCs).

  • Recognition Mechanism:

    • PTCs are identified through interactions with downstream exon junction complexes (EJCs), signaling degradation pathways.

  • Clinical Relevance: Many inherited disorders arise from mutations that introduce PTCs, making understanding NMD crucial for medical research.

9. Localization of mRNA

  • Local Translation: Impacts developmental processes and cellular functions, with mRNA localization playing a vital role in ensuring proteins are synthesized in the correct locations.

Conclusion

  • Understanding mRNA stability, degradation pathways, and processing is integral to comprehending gene expression and its regulation. Overall, the lecture emphasizes the intricate balance of mRNA stability and decay mechanisms to maintain cellular function.