JC

Viruses, Viroids, & Prions

Chapter 13: Viruses, Viroids, & Prions

Dr. Stuart

Notable Viruses

  • Hepatitis C virus
  • Coronavirus
  • Herpes virus
  • Bird flu virus
  • Smallpox virus
  • Influenza virus

Learning Objectives

  • Define viroids and prions.
  • Understand the relationship between viruses and cancer.
  • Distinguish between latent and persistent viral infections in animal cells.
  • Differentiate between lytic and lysogenic cycles of bacterial cell replication.
  • List four characteristics used to classify viruses.
  • Identify the structure of viruses.
  • Define viruses in detail.

General Characteristics of Viruses

  • Review Questions:
    • Viruses are _
    • A. Acellular
    • B. Unicellular
    • C. Multicellular
    • D. Both Unicellular & Multicellular

Molecular Structure of Microbes

  • Virus
    • Genetic Code: DNA
    • Cell Membrane: Some (from host cell)
    • Ribosomes: No
    • Nucleus: No
    • Organelles: No
  • Eukaryote
    • Genetic Code: DNA
    • Cell Membrane: Yes
    • Ribosomes: Yes
    • Nucleus: Yes
    • Organelles: Yes
  • Prokaryote
    • Genetic Code: DNA
    • Cell Membrane: Yes
    • Ribosomes: Yes
    • Nucleus: No
    • Organelles: No
  • Virion: Virus particle.

Viruses: Living or Non-Living?

  • Viruses exhibit some properties of life:
    • Homeostasis: Do not maintain a constant internal state.
    • Metabolism: Do not metabolize energy (food intake).
    • Nucleic Acid: DNA or RNA present.
    • Reproduction: Replicate only inside a host cell; no cell division.
    • Evolve: Acquire mutations and adapt over time.

Definition of a Virus

  • A virus is a tiny particle consisting of nucleic acid (DNA or RNA) and protein.
    • Some viruses possess lipids.
    • Acts as a particle that infects living organisms (hosts) and causes diseases.
  • Obligate intracellular parasites:
    • Obligate: Can only exist in a particular environment (the cell).
    • Intracellular: Occurring within a cell.
    • Parasite: Lives on/in the host cell, causing harm.
    • Unable to replicate outside of a living cell.

Characteristics of Viruses

Ecological Niche

  • Host Range:
    • The host cells a virus can infect and replicate within.
    • Typically, viruses infect specific cell types within one host species.
    • Exception: Rare cases where viruses cross host species barriers (e.g., swine to human).
    • Determined by:
    1. Specific attachment sites on host cell surfaces.
    2. Availability of host cellular factors.
    • Viruses infect organisms in:
    • Plants, Animals, Humans, Bacteria (bacteriophage), and other Viruses (virophage).

Types of Viral Infections

  • Latent Infections:
    • Dormant phase; remains in an asymptomatic host cell.
    • Not released from cells; can reactivate due to immune changes.
    • Example: Herpesviruses (Cold sores, chicken pox → shingles).
  • Persistent Viral Infections:
    • Virus is continuously active and progressively released from cells.
    • Replication occurs gradually over an extended period; often fatal.
    • Example: Hepatitis viruses (HCV, HBV) that can lead to cancer.

Structures of Viruses

Size and Morphology

  • Size of Viruses:
    • Viruses vary in size from 20 nm to over 1000 nm when measured in nanometers.
    • Not visible until the invention of the electron microscope.
  • Morphology:
    • Four basic types:
    1. Helical
    2. Polyhedral
    3. Spherical
    4. Complex

Structural Components

The Viral Nucleic Acid (Genome)

  • Genome:
    • Genetic code in the form of nucleic acid.
    • Type and number of gene segments vary across viruses.
    • Either DNA or RNA, but never both.
    • Function: Provides information to synthesize proteins for new viruses.

The Viral Protein (Capsid)

  • Capsid:
    • Composed of protein that surrounds the viral nucleic acid.
    • Functions of the capsid:
    1. Packaging of viral contents.
    2. Protecting nucleic acid from damage.
    3. Permitting transport between cells.

The Viral Envelope

  • The envelope is a lipid/protein membrane layer surrounding some viruses’ capsids.
    • Obtained from the host cell membrane during budding.
    • Functions:
    • Protects the nucleic acid
    • Assists viral entry into cells.
    • Spike proteins or glycoproteins function as keys for attachment and entry into host cells.

Taxonomy of Viruses

Classification Criteria

  • Characteristics for Virus Classification:
  1. Nucleic acids: DNA or RNA type within the virus.
  2. Particle structure: morphology of the capsid and presence/absence of an envelope.
  3. Size: dimensions of the virion.
  4. Host specificity: the type of organisms infected (plants, animals, humans, bacteria, other viruses).
  • Taxonomic Naming:
    • Family names end in –viridae.
    • Genus names end in –virus.
    • Viral species defined by shared genetic information and ecological niche.
    • Example:
    • Family: Coronaviridae
    • Genus: Betacoronavirus
    • Species: SARS-CoV
    • Subspecies: SARS-CoV-2

Viral Replication Cycle

Overview of Viral Replication

  • For multiplication, a virus must:
    • Invade a host cell.
    • Hijack the host's metabolic machinery since viruses lack their own enzymes for energy production.

Bacteriophage Replication

Lytic Cycle

  • Steps involved:
    1. Phage attaches to the host cell and injects its DNA.
    2. Phage DNA circularizes and can enter either the lytic or lysogenic cycle.
    3. New phage DNA and proteins are synthesized and assembled into viral particles (virions).
    4. Host cell undergoes lysis, releasing newly formed phage virions.

Lysogenic Cycle

  • Involves phage DNA incorporating into the host cell's DNA, termed a prophage.
    • The prophage remains latent but is replicated each time the host cell divides.
    • Occasionally, the prophage may excise from the bacterial chromosome, initiating the lytic cycle.

Animal Viruses Replication

Overview

  • Divided into two major groups:
    1. DNA viruses
    2. RNA viruses
  • Attachment to the host cell occurs through specific entry mechanisms:
    1. Receptor-mediated endocytosis
    2. Fusion (for enveloped viruses)
  • After entry, the virus capsid is uncoated, and viral replication proceeds.

DNA Viruses

  • DNA viruses replicate in the host's nucleus using host cell enzymes.
  • Viruses may have single-stranded (ss) or double-stranded (ds) DNA.
  • Synthesize capsid proteins using host cell enzymes in the cytoplasm.

RNA Viruses

  • RNA viruses replicate in the cytoplasm of the host.
  • RNA genome may further be classified as:
    • Single-stranded (ss):
      • Positive-strand ( + ) acts as mRNA for protein synthesis.
      • Negative-strand ( - ) converts to positive-strand for protein synthesis.
    • Double-stranded (ds):
      • Consist of two complementary strands of RNA.

Retroviruses

  • Retroviruses possess single-stranded RNA (ssRNA), using it to create DNA via reverse transcriptase.
  • The generated DNA integrates into the host chromosome as a provirus.
  • Example: HIV (Human Immunodeficiency Virus), a retrovirus known for causing AIDS.

Viroids and Prions

Viroids

  • Short pieces of naked RNA.
  • Lack proteins, distinguishing them from viruses.
  • Cause specific diseases such as Potato Spindle Tuber disease (PSTD).

Prions

  • Infectious protein particles; can be inherited and transmitted.
  • Distinguished proteins:
    • PrPC: normal cellular prion protein on the cell's surface.
    • PrPSc: scrapie protein that accumulates in brain cells, forming plaques.
  • Accumulation can lead to memory impairment, personality changes, and movement difficulties.
  • Example: Mad Cow Disease results from prion infection.

The Outcome of Viral Replication

  • Viruses utilize host cell machinery for replication.
  • The host cell may deploy immune responses to combat infection:
    • Mount an immune response against virus.
    • Sacrifice itself to prevent viral spread to other healthy cells.
  • Lack of control may lead to uncontrolled replication, resulting in severe consequences, including pathogenesis.

Viral Diseases

  • Viruses affecting plants and humans include:
    • Varicella Zoster virus (chickenpox/shingles)
    • Herpes virus causes cold sores.
    • Mono caused by Epstein-Barr virus.
    • Ebola virus disease.
    • Kaposi-sarcoma virus (KS).
    • Swine Influenza/Rabies Virus.