Study Notes on Viruses from Campbell Biology

Chapter 17: Viruses

General Information

  • Lecture Presentations by Kathleen Fitzpatrick, Simon Fraser University & Nicole Tunbridge, Kwantlen Polytechnic University

  • Copyright © 2025, 2020, 2016 Pearson Education, Inc. All Rights Reserved

Introduction to Viruses

  • Severe-Acute-Respiratory-Syndrome-CoronaVirus-2 (SARS-CoV-2)

    • Caused the COVID-19 pandemic, leading to over 7 million deaths worldwide by end of 2023.

    • Significantly impacted global health and economy.

  • Distinction between viruses and cells:

    • Viruses exhibit characteristics between life forms and chemical entities.

    • Size Varieties: Viruses are typically very small, measuring as tiny as 20 nm in diameter, and some being larger at up to several hundred nanometers.

Structure of Viruses

Concept 17.1: A Virus Consists of a Nucleic Acid Surrounded by a Protein Coat

  • Definition of a Virus:

    • An infectious particle consisting mainly of one or more nucleic acid molecules (either DNA or RNA) enclosed in a protein coat called a capsid.

    • Some viruses have an additional membranous envelope derived from the host's cell membrane.

Viral Genomes

  • Composition of Viral Genomes:

    • Can be double-stranded or single-stranded DNA (dsDNA or ssDNA).

    • Can also be double-stranded or single-stranded RNA (dsRNA or ssRNA).

  • Classification based on nucleic acid type:

    • DNA viruses and RNA viruses.

Capsids and Envelopes

Concept 17.1: Capsids

  • Capsid:

    • Protein shell that encloses viral genome.

    • Comprised of protein subunits called capsomeres.

    • Various structural forms of capsids exist.

Membranous Envelopes

  • Function of Membranous Envelopes:

    • Aids in host cell infections.

    • Derived from the host’s membranes with both viral and host cellular components.

Bacteriophages

  • Bacteriophages (Phages):

    • Complex viruses that specifically infect bacteria.

    • Structure includes an elongated head that contains viral DNA and a tail piece with fibers for attaching to host bacteria.

Viral Replication

Concept 17.2: Viruses Replicate Only in Host Cells

  • Obligate Intracellular Parasites:

    • Replicate exclusively within a host cell.

  • Host Range:

    • Refers to the number of host species a virus can infect.

    • Some viruses infect only a single species.

  • Viral infections generally target specific tissues in multicellular organisms.

Viral Replicative Cycles

  1. Infection Initiation:

    • Viruses bind to a host cell and introduce their genome inside.

    • Mechanisms of entry:

      • Phages utilize tail apparatus for DNA injection.

      • Other viruses can enter through endocytosis or by fusion of their envelop with the host’s membrane.

  2. Manufacture of Viral Components:

    • Infected host cells use their resources (nucleotides, enzymes, etc.) to synthesize viral proteins.

    • Capsomeres and viral nucleic acids self-assemble into new virions.

    • Virions exit the host cell, often leading to cell damage.

Types of Phage Replication Cycles

Lytic Cycle

  • Definition:

    • Phage replication cycle leading to the host cell's death.

  • Process:

    • New phages are generated, and the host cell is lysed, releasing progeny phages.

  • Designations:

    • Virulent phages reproduce solely via the lytic cycle.

Lysogenic Cycle

  1. Definition and Process:

    • Virus integrates its genome into the host's chromosome, known as prophage without destroying the host.

    • The prophage is copied during host cell division.

    • Environmental triggers can shift activity from lysogenic to lytic.

Bacterial Defense Mechanisms Against Phages

  1. Genetic Mutations:

    • Selection for bacterial mutants with altered surface proteins unfriendly to phages.

  2. Restriction Enzymes:

    • Enzymes recognize foreign DNA and degrade it; host DNA is protected via methylation.

  3. CRISPR-Cas System:

    • A system using repeated sequences to integrate phage DNA, forming a basis for immunity.

    • Transcription leads to RNA that targets and degrades matching phage DNA.

Replicative Cycles of Animal Viruses

Classification of Animal Viruses

  • Based on:

    • Nature of the viral genome (single or double stranded DNA or RNA).

    • Presence or absence of an envelope.

Infection Process

  • Entry of Enveloped Viruses:

    • Use viral glycoproteins to bind to specific receptors on host cells, initiating contact.

    • Example: Sars-CoV-2's spike protein binds to ACE2 on human cells.

  • Budding vs. Lyse:

    • Enveloped viruses can exit by budding (not always fatal to the host), whereas non-enveloped viruses typically cause lysis.

RNA as Viral Genetic Material

  1. Types of RNA Viruses:

    • Direct mRNA: Viral genome acts as mRNA for immediate protein synthesis. (Example: SARS-CoV-2)

  2. Retroviruses:

    • Use reverse transcriptase to convert RNA into DNA (Example: HIV).

    • DNA integrates into the host genome, referred to as provirus and serves multiple roles post integration.

Evolution and Pathogen Impact

  • Evolution of Viruses:

    • Considered not alive due to their dependency on host cells.

  • Origins of Viral Genomes:

    • Suggested to have evolved from plasmids and transposons.

Concept 17. 3: Pathogenic Nature of Viruses and Prions

  1. Viral Infections:

    • Damage through direct host cell disruption or via producing toxic proteins.

    • Immunological responses to infections yield common symptoms.

  2. Vaccines:

    • Provide immunity by exposing the immune system to harmless derivatives of pathogens.

    • Vaccination is critical for prevention since antibiotics do not affect viruses.

    • Antiviral drugs assist but rarely cure as in the case of HIV.

  3. Unique Viral Infections:

    • Studying receptor proteins, such as the spike protein for COVID-19, helps target therapies effectively.

Emerging Viruses

  • Definition:

    • Viruses that suddenly increase in incidence.

  • Examples include HIV, West Nile virus, Ebola virus, and more.

  • Contributing Factors:

    • Mutation rates, migration from isolated human populations, and zoonotic spillovers are primary drivers.

Viral Diseases in Plants

  1. Plant Viruses:

    • More than 2,000 viral diseases known.

    • Primarily RNA viruses with similar structures to animal viruses.

  2. Transmission Mechanisms:

    • Horizontal transmission (external sources) often facilitated by insects.

    • Vertical transmission (parental inheritance).

Prions: Infectious Proteins

Definition

  • Prions are misfolded proteins that lead to various degenerative diseases.

Transmission

  • Transmitted through contaminated food, slow-acting with decades-long incubation periods.

Mechanism

  • Misfolded forms induce normal proteins to also misfold, forming aggregates disrupting cellular function.

Research Contributions

  • Stanley Prusiner identified prions, winning the Nobel Prize in 1997.

  • Emerging evidence suggests involvement in diseases like Alzheimer's and Parkinson's.