BIOS5010_Lecture 9 miRNA and RNAi

Gene Expression and Its Control

Overview

The lecture by Dr. Jerome Korzelius at the University of Kent, titled "Gene Expression and its Control," delves into the intricacies of gene expression, focusing particularly on microRNAs (miRNAs) and RNA interference (RNAi).

Central Dogma of Molecular Biology

Definition: The Central Dogma describes the flow of genetic information in biological systems, specifically from DNA to RNA to protein. It encompasses processes like replication, transcription, and translation.
- Replication: The duplication of DNA.
- Transcription: The process of copying a segment of DNA into RNA.
- Translation: The synthesis of proteins based on the RNA sequence.
- Reverse Transcription: The conversion of RNA back to DNA.

Discrepancies Between mRNA and Protein Levels

Correlation Issues

It has been noted that protein and mRNA levels do not always align. Factors such as heat stress, infection, and developmental stages influence this correlation, affecting the production and turnover of mRNA and proteins differently.
Bacterial Example: Studies indicate that environmental conditions can lead to gene-specific changes in decay rates of mRNA. For example, rapidly growing bacteria exhibit a transcriptome-wide rapid decay of mRNA, while slowly growing ones demonstrate slower decay.

Mechanisms of mRNA Regulation

Control Mechanisms

Qualitative Control: Multiple protein products can arise from one primary mRNA transcript through splicing.
Quantitative Control: The level of mRNA present is regulated to control protein synthesis. Mechanisms include mRNA decapping or deadenylation, which modulate mRNA stability and availability for translation.

Lecture Focus: Small RNAs

Overview of Today's Topic

This lecture focuses on:

The discovery and function of small non-coding RNAs in gene silencing
The mechanisms of RNA interference and the roles of siRNA and miRNA
The significance of miRNA abundance in eukaryotic cells and how RNAi serves as a tool for studying gene function.

Two Classes of Non-Coding RNAs

Long Non-Coding RNAs (>200 bp): Related to transcriptional silencing, primarily occurring within the nucleus.
Short Non-Coding RNAs (20-30 bp): Involved in post-transcriptional gene silencing in the cytoplasm.

Discovery of Small Non-Coding RNAs

The concept of serendipity in scientific research is illustrated, suggesting that important findings often come unexpectedly.

Applications in Plant Genetics

The text explores traditional genetics in breeding new flower varieties, contrasting this with the introduction of transgenic methods since the 1980s.
Example: Increased expression of the chalcone synthase gene resulted in deeper purple petunias.

Transgene Induced Gene Silencing (TIGS)

A fascinating concept where the introduction of a transgene can lead to silencing of the endogenous gene, causing unexpected outcomes such as flower color changes.

RNA Interference (RNAi) Mechanism

Discovery of RNAi

RNAi is initiated through the introduction of double-stranded RNA (dsRNA), highlighting its specificity for targeting homologous mRNA without affecting other transcripts. This process allows degradation of specific mRNA.

Nobel Prize Recognition

The phenomenon of RNA interference, particularly in the context of bilateral function across various biological applications, won the Nobel Prize in Physiology or Medicine in 2006.

RNAi Definitions

RNA Interference (RNAi): This process modulates mRNA expression through interference from small interfering RNAs (siRNAs) derived from longer dsRNAs.
MicroRNAs (miRNAs): Endogenous small RNAs that regulate gene expression through pathways similarly to siRNAs.

Steps in siRNA/miRNA-Mediated Gene Silencing

Key Steps

Processing by Drosha: miRNAs originate as larger pri-miRNA precursors that are processed into pre-miRNAs.
Dicer's Role: Cleaves these precursors into short siRNA molecules, crucial for the RNA interference process.
Formation of RISC: The RNA-induced silencing complex is formed to incorporate siRNAs, delivering these molecules to target mRNAs.
mRNA Destabilization: Binding of the RISC complex often leads to degradation or inhibition of mRNA, effectively silencing gene expression.

Differences Between siRNA and miRNA

Binding Characteristics: miRNAs tend to bind to partially complementary sites, affecting translation, while siRNAs typically bind fully to their target mRNAs, triggering degradation through AGOs.

Importance in Eukaryotic Biology

Eukaryotes carry numerous miRNAs that regulate a vast array of mRNAs throughout different developmental stages. Each miRNA can influence hundreds of target mRNAs, showcasing their extensive role in gene regulation.

Applications of RNAi in Research

RNAi technology is being harnessed in various models, such as C. elegans, proving efficient for gene function studies, allowing manipulation through diverse methods of introduction and spreading of RNAi effects.

Key Takeaways

siRNAs and miRNAs play significant roles in regulating gene expression by modulating mRNA stability and translation.
Understanding the pathways of RNA silencing provides important insights into gene function, applicable in research and therapeutic contexts.