Lecture+31++Viruses+Sp+25

Announcements

  • Exam 3: Scheduled for Monday night at 7 pm covering Lectures 20 – 30.

  • Current Top Hat scores are available in Canvas; total possible points = 50, with additional points for exceeding that.

  • We are entering the final section of the class focusing on Viruses and Metabolism (Lectures 31 - 37).

  • Note: Bonus Lectures 38 and 39 will not be on the final exam, though they will contribute to Top Hat points.

Viruses Overview

  • Definition: Viruses are infectious particles composed of genes packaged in proteins and optionally a membrane.

    • Non-Living: Cannot reproduce or conduct metabolism outside of a host.

    • Individual Viral Particle: Known as a virion.

    • Example: MERS-CoV.

Types of Viruses

  • Viruses can infect all living organisms:

    • Plants: Viroids.

    • Animals: Viruses.

    • Fungi: Mycoviruses.

    • Bacteria: Bacteriophages.

    • Archaea: Viruses.

The Origin of Viruses

  • The origin of viruses is unclear, but three hypotheses exist:

    1. Virus-First Hypothesis: Simple ancient organisms evolved into more complex infectious forms.

    2. Progressive Hypothesis: Certain genes cluster together to create a malicious code that escapes cell boundaries.

    3. Regressive Hypothesis: Suggests some viruses originate from once-free living cells that have lost their functions.

Viral Structural Features

  • Genomes:

    • DNA Viruses: Can have single-stranded (ssDNA) or double-stranded (dsDNA) genomes.

    • RNA Viruses: May have ssRNA or dsRNA genomes.

    • Types of ssRNA:

      • + ssRNA: Acts as mRNA.

      • - ssRNA: Complementary to mRNA.

Replication of DNA Viruses

  • Utilizes host cell DNA polymerase to replicate many copies.

  • The replication process includes transcription to create viral mRNA using host cell RNA polymerase.

Replication of RNA Viruses

  • Involves Viral RNA and the enzyme Viral Replicase to produce additional viral RNA.

  • For some RNA viruses (e.g., retroviruses), reverse transcription occurs where the viral RNA is converted to DNA, which integrates into the host genome to facilitate continuous expression.

Capsid and Viral Envelope

  • Capsid: Protein shell surrounding the virus or its genome; composed of protein subunits called capsomeres.

    • Example Viruses: Tobacco Mosaic Virus (first discovered virus), and Adenovirus (causes respiratory illnesses).

  • Viral Envelope: A membrane comprised of host cell-derived phospholipids and proteins, along with unique viral proteins.

    • Example Viruses: Influenza Virus (causes flu) and Human Immunodeficiency Virus (HIV; causes AIDS).

Host Range of Viruses

  • Definition: Refers to the variety of cell types or species a virus can infect, determined by viral surface proteins and corresponding host cell receptors.

  • Adsorption: The process of the virus attaching to the host cell membrane.

Examples of Host Range

  • Influenza A Virus: Illustrates persistence in diverse species and zoonotic diseases that can transmit between animals and humans.

Bacteriophages

  • Definition: Viruses that specifically infect bacteria are primarily DNA viruses (e.g., dsDNA).

  • Types:

    • Virulent Phage: Enter the lytic cycle of infection, leading to cell lysis.

    • Temperate Phage: Can undergo either lytic or lysogenic cycles; the choice may depend on environmental conditions.

Viral Infections and Cellular Mechanism

  • Viruses hijack host cellular machinery including proteins and organelles to replicate their components.

  • Retrovirus Example: HIV uses its own reverse transcriptase to convert viral RNA to DNA incorporating it into the host genome, facilitating persistent expression of