Recording-2025-03-16T20:30:07.628Z Lecture 13.6

RNA Genomes Overview

• RNA genomes can be classified into different categories based on their structure and function:

  • Double-Stranded RNA (dsRNA)

  • Single-Stranded RNA (ssRNA)

    • Positive-Sense RNA (pRNA)

    • Negative-Sense RNA (nRNA)

    • Retroviruses (technically positive-sense, but behave differently)

Central Dogma of Molecular Biology

• Central concept in molecular biology describing the flow of genetic information:

  • DNA Replication: DNA can replicate itself to create more DNA.

    • Process is known as DNA Replication.

  • Transcription: DNA is used as a template to synthesize RNA.

    • This process is called Transcription.

  • Gene Expression: RNA is used as a template to synthesize proteins.

    • RNA = messenger RNA (mRNA), which is essential for protein synthesis.

Virus Infections and Information Flow

• Viruses also utilize the central dogma but often require host cell machinery for replication and transcription.

  • DNA Viruses (e.g., Herpes Virus):

    • Utilize host cell enzymes for DNA replication and transcription to produce mRNA for protein synthesis.

  • RNA Viruses require special mechanisms:

    • As RNA viruses enter the cell, they must first establish a replication and transcription process.

RNA Virus Strategies

• RNA viruses cannot utilize existing cellular machinery for RNA replication, thus they require unique pathways:

  • RNA-Dependent Transcription:

    • RNA viruses direct the host cell to synthesize more RNA using their existing RNA as a template.

  • Reverse Transcription:

    • Some RNA viruses (retroviruses) convert their RNA into DNA, creating a complementary DNA strand that can integrate into the host genome.

Enzymatic Dependence in Viruses

• Viruses must package certain enzymes necessary for unique replication steps as host cells lack these pathways:

  • RNA-Dependent RNA Polymerase: Needed for RNA viruses.

  • Reverse Transcriptase: Needed for retroviruses to convert RNA to DNA.

Messenger RNA Production

• All viruses must produce mRNA to facilitate the creation of proteins, such as:

  • Protein Capsid: Essential structural component of viruses.

Pathways for Different Types of RNA Viruses

• Positive-Sense RNA Viruses (pRNA):

  • Can be directly translated into proteins as they function like mRNA.

  • Must replicate RNA templates to generate more viral RNA.

  • First proteins made include those that enable replication.

• Negative-Sense RNA Viruses (nRNA):

  • Must serve their RNA directly as templates to synthesize mRNA.

  • Following mRNA translation, they replicate their negative-sense RNA using newly formed mRNA as templates.

• Retroviruses:

  • Utilize positive-sense RNA to synthesize DNA using reverse transcription.

  • The resulting DNA undergoes transcription to produce viral RNA and proteins, enabling further replication and infection.

Amplification of Viral RNA

• RNA viruses can exponentially amplify their replication:

  • Example flow: 1 pRNA can generate multiple copies of nRNA and subsequently many pRNA.

  • Allows for rapid proliferation of viral particles within an infected host.

Summary of Viral Replication

• Viral replication entails unique adaptations, relying on specific enzymatic processes for RNA production:

  • Positive-sense viruses need only translation for mRNA production.

  • Negative-sense viruses require a transcription step to generate mRNA.

  • Retroviruses reverse the flow of the central dogma by incorporating their genetic material into the host's DNA.