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.