Characterizing and Classifying Viruses, Viroids, and Prions

Characterizing and Classifying Viruses, Viroids, and Prions

Introduction

Viruses, viroids, and prions are unique infectious agents that differ fundamentally from cellular life forms. This chapter explores their characteristics, classification, and replication strategies, providing foundational knowledge for microbiology students.

Characteristics of Viruses

General Properties

Viruses are acellular entities that require host cells for replication and metabolic activity. They are responsible for many diseases affecting humans, animals, plants, and bacteria.

• Acellular Structure: Viruses lack cellular components such as plasma membrane, cytosol, and organelles.

• Genetic Material: Each virus contains either DNA or RNA, but never both.

• Obligate Intracellular Parasites: Viruses cannot carry out metabolic pathways or reproduce independently; they must hijack host cell machinery.

• States: Viruses exist in both extracellular (virion) and intracellular states.

• Host Range: Viruses infect a wide variety of hosts, including humans, animals, plants, and bacteria.

Example: The influenza virus infects human respiratory cells, causing seasonal flu epidemics.

Host Specificity

Viruses display remarkable specificity for their host cells, determined by interactions between viral surface proteins and host cell receptors.

• Specificity: Most viruses infect only particular host cells due to the affinity of viral surface proteins for complementary proteins on host cell surfaces.

• Narrow Host Range: Some viruses are so specific they infect only a particular kind of cell within a particular host.

• Broad Host Range: Other viruses (generalists) infect many kinds of cells or multiple hosts.

• Universal Susceptibility: All types of organisms are susceptible to some virus.

Example: Bacteriophages infect only specific strains of bacteria, while rabies virus can infect many mammalian species.

Size and Morphology of Viruses

Size Comparison

Viruses are much smaller than cells and other cellular structures. Their size can range from about 10 nm to over 500 nm.

• Examples:

  • Poliovirus: 30 nm

  • Bacteriophage MS2: 24 nm

  • Bacteriophage T4: 50 nm x 225 nm

  • Smallpox virus: 200 nm x 300 nm

  • Tobacco mosaic virus: 15 nm x 300 nm

  • Bacterial ribosome: 25 nm (for comparison)

  • Red blood cell: 7,000 nm diameter

  • E. coli bacterium: 1,000 nm x 3,000 nm

Additional info: Viruses are generally too small to be seen with light microscopes and require electron microscopy for visualization.

Capsid and Morphology

The capsid is the protein shell that encases the viral genome, composed of protein subunits called capsomeres.

• Capsid Shapes: Common shapes include helical, polyhedral (icosahedral, with 20 sides), and complex forms.

• Functions: The capsid protects viral nucleic acid and aids in attachment to host cells.

Viral Envelope

Some viruses possess an envelope derived from the host cell membrane during viral replication or release.

• Composition: The envelope consists of a phospholipid bilayer and proteins, including virally encoded glycoproteins (spikes).

• Role: Spikes play a key role in host recognition and attachment.

• Enveloped vs. Nonenveloped: Viruses with an envelope are called enveloped virions; those without are naked virions.

• Fragility: Enveloped viruses are generally more fragile than naked viruses but can evade the immune system more effectively.

Are Viruses Alive?

Comparison of Viruses and Cells

The question of whether viruses are alive is debated. Viruses possess some characteristics of life but lack others.

Property	Viruses	Cells
Metabolism	None outside host cell	Active
Growth	Do not grow	Grow and divide
Cellular Structure	Acellular	Cellular
Genetic Material	DNA or RNA	DNA (in most)
Genome Size	Small, few genes	Large, many genes
Replication	Assembly-line using host enzymes	Self-replicating

Additional info: Viruses are considered obligate intracellular parasites and are only "active" when inside a host cell.

Classification of Viruses

Criteria for Classification

Viruses are classified based on several key features:

• Type of Nucleic Acid: DNA or RNA, single-stranded (ss) or double-stranded (ds), linear or circular.

• Presence of Envelope: Enveloped or nonenveloped (naked).

• Capsid Shape: Helical, polyhedral, or complex.

• Host Range: Type of host infected (animal, plant, bacterium).

Additional info: Viruses are organized into families; higher-level relationships among viruses are still being studied by taxonomists.

Replication of Viruses

Bacteriophage Replication: Lytic Cycle

The lytic cycle is a replication process in which bacteriophages produce new virions, resulting in host cell death and lysis.

• Steps of the Lytic Cycle:

  1. Attachment

  2. Entry

  3. Synthesis

  4. Assembly

  5. Release (lysis)

• Burst Size: The number of virions released per lysed cell.

• Burst Time: The time required for a single cycle of infection and lysis.

Bacteriophage Replication: Lysogenic Cycle

Some bacteriophages undergo a lysogenic cycle, integrating their genome into the host cell's DNA and replicating without immediate lysis.

• Prophage: The inactive phage genome integrated into the host DNA.

• Lysogenic Conversion: Phages may carry genes that alter the phenotype of the host cell.

• Induction: The prophage may later exit the host genome and enter the lytic cycle.

Replication of Animal Viruses

Animal viruses follow similar basic replication pathways as bacteriophages, with differences due to the presence of an envelope, the eukaryotic nature of animal cells, and the absence of a cell wall.

• Attachment: Mediated by glycoprotein spikes or other molecules; animal viruses do not have tails or tail fibers.

• Entry Mechanisms:

  • Membrane fusion (enveloped viruses)

  • Endocytosis

  • Direct penetration

• Uncoating: Removal of the capsid to release the viral genome before replication.

Synthesis of Animal Viruses

The synthesis of viral components depends on the type of nucleic acid present in the virus.

• DNA Viruses: Often enter the nucleus; replication may resemble cellular DNA replication.

• RNA Viruses: Replicate in the cytoplasm; strategies differ based on whether the RNA is positive-sense (+ssRNA) or negative-sense (-ssRNA).

• Key Questions:

  • How is mRNA synthesized?

  • What serves as the template for nucleic acid replication?

Additional info: (+)ssRNA can act directly as mRNA, while (-)ssRNA must be transcribed into (+)ssRNA before translation.

Summary Table: The Novel Properties of Viruses

Feature	Viruses	Cells
Metabolism	None outside host cell	Active
Growth	Do not grow	Grow and divide
Cellular Structure	Acellular	Cellular
Genetic Material	DNA or RNA	DNA (in most)
Genome Size	Small, few genes	Large, many genes
Replication	Assembly-line using host enzymes	Self-replicating