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Bioscience 2_Chapter 21 and 42 Part 2_JKele

Chapter Overview

  • Chapter: Blood Typing, Immunology, and Viruses

  • Course: BIOL 122: Bioscience II

  • Instructor: Joseph Kele, Ed.Dc., M.S.

  • Semester: Spring 2025

  • Information Source: Openstax Biology 2e

What is a Virus?

  • Definition: Infectious particle consisting of genes packaged in a protein coat.

  • Function: Viruses require host cells to grow and reproduce.

  • Comparison to Complement Proteins: Complement proteins have a role in immune response to viruses.

Difference Between Viruses and Bacteria

  • Viruses:

    • Not living organisms.

    • Only reproduce inside host cells.

    • Dormant outside living cells.

    • Submicroscopic in size.

  • Bacteria:

    • Living organisms, single-celled.

    • Can generate energy, make food, move, and reproduce (binary fission).

    • Found in diverse environments: soil, water, plants, and human bodies.

    • Larger size compared to viruses.

Infection Types

  • Viral Infections: Systemic infections; can include diseases such as influenza, measles, polio, AIDS, COVID-19.

  • Bacterial Infections: Typically localized infections; examples include pneumonia, tuberculosis, tetanus, food poisoning.

Viral Structure

  • Capsid: Protein shell enclosing the viral genome.

  • Viral Genome Types:

    • Double-stranded DNA or RNA.

    • Single-stranded DNA or RNA.

  • Capsomeres: Protein subunits that make up the capsid.

  • Capsid Shapes: Rod-shaped, polyhedral, spherical.

Types of Viruses

  • Helical Viruses: Viral nucleic acid coiled; example: Tobacco Mosaic Virus.

  • Polyhedral Viruses: Nucleic acid in a polyhedral shell; example: Adenovirus (causes mild cold or flu).

  • Spherical/Icosahedral Viruses: Example: Influenza virus.

  • Complex Viruses: Incorporate a variety of components; example: Bacteriophage.

Viral Envelopes

  • Some viruses possess membranous envelopes made from host cell membranes.

    • Contains phospholipids and membrane proteins from the host and viral glycoproteins.*

Bacteriophages

  • Complex viruses that infect bacteria.

  • Example: T2, T4, T6 bacteriophages infect E. coli.

  • Structure: Elongated icosahedral heads with DNA and a protein tail for attachment.

Host Ranges

  • Host range: Limited number of host species a virus can infect.

  • Mechanism: Viruses identify specific host cells via receptor interactions.

  • Example: West Nile and equine encephalitis infect various hosts, whereas measles only affects humans.

Steps of Virus Infections

  1. Attachment: Binding to host cell receptors.

  2. Entry: Viral genome enters host cells; mechanisms include endocytosis or membrane fusion.

  3. Replication and Assembly: Viral genomes direct synthesis of new viruses.

  4. Egress (Release): Can involve cell lysis or budding without killing the cell.

Detailed Steps of Virus Infections

Step 1: Attachment

  • Viruses attach to cells via specific receptor interactions with capsid proteins or glycoproteins.

Step 2: Entry

  • For bacteriophage: DNA enters directly.

  • For other viruses: May enter via endocytosis or fusion.

Step 3: Replication and Assembly

  • Process varies for different genome types: DNA or RNA.

  • Includes transcription of RNA and duplication of viral genomes.

Step 4: Egress (Release)

  • Involves host cell lysis or budding to release new viruses into the environment.

Viral Replicative Cycles

Lytic Cycle

  • Culminates in host cell death; phages rapidly replicate, leading to cell lysis.

Lysogenic Cycle

  • Phages integrate with host DNA; prophages can replicate without destroying host.

The Lytic and Lysogenic Cycles of Phage λ

  • Description of dual pathways of phage λ within host cells.

  • Phages can replicate via both cycles with different outcomes.

RNA Viruses

  • Effective in infecting animal cells with various RNA genome types.

    • RNA can function as mRNA or as a template for mRNA synthesis.

    • Retroviruses have reverse transcriptase for RNA to DNA conversion.

HIV: Human Immunodeficiency Virus

  • A retrovirus causing AIDS; integrates its RNA into host DNA.

Replicative Cycle of Enveloped RNA Viruses

  • Involves specific receptor binding and subsequent integration and assembly processes.

Plant Viruses

  • Transmission types: Horizontal (via damaged tissues or vectors) and Vertical (from parent plant).

Common Symptoms of Plant Viral Diseases

  • Hyperplasia, hypoplasia, discoloration, abnormal growth patterns.

Viroids: Small Circles of RNA

  • Infect plants; do not manufacture proteins.

Vaccines for Viral Infections

  • Trigger immune responses using attenuated/live viruses or killed viruses.

Prions: Proteinaceous Infectious Particles

  • Lack nucleic acids; cause neurodegenerative diseases such as mad cow disease and Creutzfeldt-Jakob disease.