Study Notes on Viruses, Viroids, and Prions

Virus: Overview

  • Viruses vary in both size and shape. Examples include:

    • Poxvirus: 250 nm

    • Herpes simplex virus: 150 nm

    • Flavivirus: 22 nm (causes fever)

    • HIV: 100 nm

    • Influenza virus: 100 nm

    • Adenovirus: 75 nm

    • Poliovirus: 30 nm

    • Rabies virus: 110 nm

    • T2 bacteriophage: 65 nm

  • Comparison to other organisms (for scale):

    • E. coli (Bacterium): Approximately 2 μm2 \text{ μm} long.

    • Yeast cell (Eukaryote): Around 7 μm7 \text{ μm} long.

Virus Composition

  • Viruses are considered alive only when they are inside a host cell.

  • Composition:

    • Contain either DNA or RNA (not both).

    • Have a protein coat called the capsid.

    • Some have an envelope made of lipids, proteins, and carbohydrates, often with spikes.

  • Complete virus particles (virions) are ready to infect host cells.

Viral Characteristics

  • Multiplication:

    • Viruses reproduce by taking over the host cell's machinery.

    • Not sensitive to antibiotics.

    • Sensitive to interferon, a protein that helps protect cells from viruses.

  • Host Range:

    • Most viruses infect only one particular type of host (tropism).

    • Bacteriophages are viruses that infect bacteria, considered a potential alternative to antibiotics.

Virus Structure

  • Components:

    • Nucleic Acid: Can be linear or circular, single-stranded (SS) or double-stranded (DS), and either DNA or RNA, ranging from a few thousand to 250,000250,000 nucleotides.

    • Capsid: A protein shell made of capsomers that encases the nucleic acid.

    • Envelope: Optional lipid bilayer that surrounds the capsid, often studded with spikes to aid in host recognition and attachment.

Immune Response to Virus

  • The host immune system produces antibodies in reaction to viral surface proteins (antigens).

  • Viruses can mutate rapidly, leading to multiple infections by the same type of virus (e.g., influenza).

General Virus Morphology

  • Types of Structure:

    • Helical Viruses: Long rods with a hollow center (e.g., Ebola).

    • Polyhedral Viruses: Often icosahedral shapes (e.g., Polio virus).

    • Envelope viruses: Roughly spherical shape (e.g., Herpes virus).

    • Complex Viruses: Combine features, like bacteriophages.

Virus Taxonomy

  • Viral taxonomies are categorized based on shared characteristics (e.g., genetic information, morphology, host range).

  • Animal viruses grow within:

    • Living Animals (e.g., rabbits, mice)

    • Embryonated eggs

    • Cell cultures: Primary or continuous/immortal cell lines (from cancer cells like HeLa). Viral growth often causes cytopathic effects (CPE).

Viral Multiplication Cycles

  • General Animal Virus Replication Cycle:

    1. Adsorption (Attachment)

    2. Penetration (Entry)

    3. Uncoating

    4. Biosynthesis

    5. Maturation (Assembly)

    6. Release

  • Lytic Cycle (Bacteriophages):

    • The viral life cycle where the virus attaches, penetrates, replicates, assembles, and causes the host cell to lyse (burst).

    • Steps: Attachment, Penetration, Biosynthesis, Maturation, Release.

  • Lysogenic Cycle (Temperate Bacteriophages):

    • The phage DNA (prophage) integrates into the bacterial chromosome and can remain dormant, replicating with the host genome.

    • The latent virus can become active (induction), switching to the lytic cycle, leading to new virions and cell lysis. In animal viruses, this is called a provirus.

Viral Induced Diseases

  • Viral infections can cause cancer (viral oncogenesis); approximately 10%10\% - 15%15\% of human cancers are virus-induced.

  • Examples of Oncogenic viruses:

    • Adenoviridae: Respiratory diseases (some oncogenic potential in animals).

    • Poxviridae: Pus-filled lesions (some tumor formation in animals).

    • Herpesviridae: Epstein-Barr Virus (EBV) linked to lymphomas; Kaposi's Sarcoma-associated Herpesvirus (KSHV); Herpes Simplex Virus (HSV).

    • Human Papillomaviruses (HPVs): High-risk types (16,1816, 18) cause cervical and other cancers.

    • Hepatitis B (HBV) and C (HCV): Causes hepatocellular carcinoma.

Viroids and Prions

  • Viroids: Small, circular, naked RNA molecules that cause plant diseases (e.g., Potato Spindle Tuber Viroid (PSTV)); do not code for proteins.

  • Prions: Misfolded proteins (PrP^Sc^) that cause neurodegenerative diseases (transmissible spongiform encephalopathies - TSEs) by templating normal proteins (PrP^C^) to misfold (e.g., mad cow disease, Kuru, Creutzfeldt-Jakob disease).

Covid-19 Overview

  • SARS-CoV-2, a large enveloped RNA virus, causes COVID-19. It has proofreading abilities that reduce mutation rates.

  • Originates from bats (possibly pangolins); transmitted via respiratory droplets/aerosols.

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