Chapter 13 Viruses, Viroids, and Prions

INTRODUCTION TO VIRUSES

  • General Structure of Viruses

    • Viruses are obligate intracellular parasites of either DNA or RNA encapsulated in a protein coat.

    • Capsid: The protein coat that shields nucleic acids from environmental toxins and enzymes.

    • Nucleocapsid: The combination of nucleic acids and the capsid.

    • Spikes: Surface proteins that facilitate viruses in binding to specific host cell receptors to initiate infection.

  • Virions: A complete virus particle released from the host cell into the extracellular space (outside the cell).

ENVELOPED VS. NON-ENVELOPED VIRUSES

  • Some viruses possess an outer lipid bilayer referred to as an envelope.

  • Enveloped Viruses:

    • Characterized by having an outer lipid bilayer obtained from the host cell.

    • A matrix protein connects the envelope to the nucleocapsid.

    • Susceptibility: Enveloped viruses are vulnerable to soaps and detergents that damage the envelope, rendering the virus non-infectious.

    • Example: COVID-19 virus is enveloped, which is why handwashing with soap can reduce disease transmission.

  • Non-Enveloped (Naked) Viruses:

    • Do not possess an outer lipid bilayer.

COMMON SHAPES OF VIRUSES

  • Viruses can present in various shapes, the three most common are:

    1. Icosahedral: Spherical viruses made of 20 triangular faces (like a soccer ball).

    2. Helical: Cylindrical viruses with capsids arranged in a helical form (like a spiral staircase).

    3. Complex: Viruses with variations in structure that may include multiple shapes (ex. phages).

BACTERIOPHAGE INFECTIONS

  • Bacteriophages (Phages): Viruses that infect bacterial cells and can facilitate horizontal gene transfer.

PRODUCTIVE VS. LATENT STATE INFECTIONS

  • Phage infections can be categorized into two main types:

    1. Productive State:

    • New phage particles are produced.

    • Can result in cell lysis upon virus release or the host cell may continue to live and slowly release viruses.

    1. Latent State:

    • Viral genome integrates into the host chromosome without producing new phages.

    • Integration may alter the host cell's characteristics; the viral genome replicates when the host cell multiplies.

LYTIC AND LYSOGENIC BACTERIOPHAGES

  • Lytic (Virulent) Phages: Cause productive infections that lead to death of the host cell by lysis.

  • Lysogenic (Temperate) Phages: Undergo productive infections and can enter a latent state.

    • Phage DNA integrates into the host chromosome, forming a prophage.

    • A lysogen is a bacterial cell containing a prophage, which can remain integrated indefinitely, or can excise to initiate a lytic infection.

LYTIC PHAGE INFECTIONS CYCLE

  • The lytic cycle involves a series of steps resulting in the destruction of the host cell:

    1. Recognition and Attachment: Phage identifies and binds to the host cell.

    2. Entry of the Phage Genome: Injection of viral genome into the cell.

    3. Gene Expression: Phage genes are transcribed and translated.

    4. Assembly: New phage particles are constructed.

    5. Release: Lysis of the host cell releases new phages.

PLAQUE ASSAYS

  • Plaque Assay: A method for quantifying phage particles in a sample by observing the formation of plaques, which are clear areas indicating cell death due to phage infection.

    • Each plaque represents a Plaque-Forming Unit (PFU) from a single phage particle.

INTRODUCTION TO ANIMAL VIRUS INFECTIONS

  • Similar to phage infections, animal virus infections also follow a series of steps including:

    1. Attachment: Virus binds to host cell receptors.

    2. Entry and Uncoating: Viral genome is released into the cytoplasm.

    3. Synthesis: Viral components are produced.

    4. Assembly: New virions are assembled.

    5. Release: New virions exit the host cell.

ENTRY METHODS FOR ENVELOPED AND NON-ENVELOPED VIRUSES

  • Enveloped viruses can enter cells via:

    1. Membrane Fusion: The viral envelope fuses with the host cell’s membrane.

    2. Endocytosis: The virus is engulfed in an endocytic vesicle.

  • Non-enveloped viruses can strictly enter through endocytosis.

SYNTHESIS & REPLICATION OF ANIMAL VIRUSES

  • There are necessary events for complete viral replication:

    1. Synthesis of Structural Proteins and viral enzymes.

    2. Replication of the Viral Genome (which can vary depending on the type of viral genome).

CLASSIFICATION OF ANIMAL VIRUSES BY GENOME TYPE

  • Animal viruses can have diverse types of genomes influencing viral synthesis and replication:

    1. (+/−) dsDNA: Double-stranded DNA is transcribed and replicated according to the central dogma of molecular biology.

    2. (+) ssRNA: Single-stranded RNA is directly translated by host ribosomes.

    3. (−) ssRNA: Used as a template to create (+) ssRNA.

COVID-19 VIRUS (SARS-CoV-2)

  • COVID-19: Discovered in late 2019; caused severe lung issues attributed to SARS-CoV-2.

  • SARS-CoV-2 is an enveloped (+ssRNA) virus with spike proteins that resembles a corona.

  • Transmission occurs via respiratory droplets; its infection can lead to severe respiratory illness.

VIROIDS

  • Viroids: Obligate intracellular entities made solely of a single, short strand of ssRNA arranged in a circular form.

  • They do not code for any proteins but act similarly to siRNAs, interfering with the expression of plant genes and causing disease.

PRIONS

  • Prions: Misfolded proteins that induce misfolding of normal proteins, leading to brain diseases characterized by spongiform encephalopathies.

  • Examples include: Scrapie in sheep, transmissible spongiform encephalopathies (TSEs) in other animals.