Characterizing and Classifying Viruses, Viroids, and Prions

General Characteristics of Viruses

Are Viruses Alive?

Viruses are unique infectious agents that exist at the boundary between living and non-living entities. They are considered obligate intracellular parasites because they require a host cell to reproduce and cannot carry out metabolic processes independently.

• Genetic Material: Viruses contain either DNA or RNA, but never both.

• Protein Coat: Their genetic material is surrounded by a protein coat called a capsid.

• Replication: Viruses hijack the host cell's machinery to replicate and synthesize new viral particles.

• Transfer Structures: They produce structures to transfer their nucleic acid to other cells.

Why can’t an antiviral drug inhibit viral multiplication? Most antiviral drugs target specific steps in viral replication, but the high mutation rates and reliance on host cell machinery make it difficult to develop broad-spectrum antivirals without harming host cells.

Extracellular and Intracellular States

• Extracellular State (Virion): The virus exists as a complete particle with a capsid and, in some cases, an envelope. The outermost layer provides protection and recognition sites for host cells.

• Intracellular State: The capsid is removed, and the virus exists as nucleic acid within the host cell.

Structure of Viruses

Nucleic Acid

Viral genomes can be composed of DNA or RNA, which may be single-stranded or double-stranded, and can be linear or circular.

• DNA Viruses: May be single- or double-stranded.

• RNA Viruses: May be single- or double-stranded.

Capsid and Envelope

The capsid is a protein shell that surrounds and protects the viral nucleic acid. It is composed of protein subunits called capsomeres. Some viruses possess an additional envelope derived from the host cell membrane, which may contain viral proteins and glycoprotein spikes.

• Envelope: Composed of a phospholipid bilayer and proteins, often with spikes that aid in host recognition and attachment.

• Antigenic Variation: The envelope and spikes are highly mutable, which can hinder immune recognition.

Viral Morphology

Shapes of Viruses

• Helical: Cylindrical capsid with nucleic acid inside.

• Polyhedral: Many-sided, often icosahedral (20 faces).

• Enveloped: Spherical, with helical or polyhedral nucleocapsid inside.

• Complex: Capsids with additional structures, such as bacteriophages.

Host Range and Specificity

Viruses can infect all forms of life, but most are specific to a particular host species or cell type. This specificity is determined by the virus's ability to attach to and utilize the host's cellular machinery.

• Bacteriophages: Viruses that infect bacteria.

• Therapeutic Potential: Host range specificity and cell-killing properties can be harnessed for therapies, such as targeting bacteria or tumors.

Size Comparison

Viruses are much smaller than cells and organelles, typically ranging from 20 to 300 nm in diameter.

Unique Properties of Viruses

Viruses differ from cells in several fundamental ways, as summarized in the table below.

Viruses	Cells
Inert outside of host cell	Metabolize on their own
Obligate intracellular parasites	Cellular
Contain either DNA or RNA	Contain both DNA and RNA
Much smaller (10–300 nm)	200 nm to 12 cm or larger
Protein capsid, sometimes envelope	Surrounded by a phospholipid membrane and, in plants/fungi, a cell wall
Replicate only inside host cells	Self-replicating by asexual/sexual means

Classification of Viruses

Viruses are classified based on:

• Type of nucleic acid (DNA or RNA)

• Presence or absence of an envelope

• Shape (helical, polyhedral, complex)

• Size

Viral genera are grouped into families, but relationships among viruses are not fully understood.

Viral Replication in Bacteria

Lytic and Lysogenic Cycles

Bacteriophages can replicate via two main cycles:

• Lytic Cycle: Results in host cell lysis and death.

• Lysogenic Cycle: Phage DNA integrates into host genome and replicates with it, remaining latent until induced to enter the lytic cycle.

Five Stages of Lytic Cycle

1. Attachment

2. Entry

3. Synthesis

4. Assembly

5. Release

Lysogenic Cycle

During lysogeny, the phage genome integrates into the host chromosome as a prophage. The host cell divides normally, passing the prophage to daughter cells. The prophage can later excise and initiate the lytic cycle.

Consequences of Lysogeny

• Lysogenic cells are immune to infection by the same phage type.

• Phage conversion can confer new properties, such as toxin production.

• Specialized transduction allows transfer of specific bacterial genes to new hosts.

Viral Multiplication in Animal Cells

Attachment and Entry

• Attachment: Chemical attraction between viral proteins (often glycoprotein spikes) and host cell receptors.

• Entry Mechanisms:

  • Direct penetration

  • Receptor-mediated endocytosis

  • Membrane fusion (for enveloped viruses)

• Uncoating: Removal of capsid to release viral genome.

Synthesis and Assembly

• DNA Viruses: Replicate in the nucleus; proteins synthesized in cytoplasm.

• RNA Viruses: Replicate in the cytoplasm; positive-sense RNA acts as mRNA, negative-sense RNA must be transcribed to positive-sense before translation.

Release

• Budding: Enveloped viruses acquire their envelope from the host cell membrane.

• Cell Rupture: Non-enveloped viruses are released by cell lysis, causing cell death.

Retroviruses

• Use reverse transcriptase to copy RNA into DNA, which integrates into the host genome as a provirus.

• The provirus is permanent and cannot be excised.

Latent and Persistent Viral Infections

• Latent Infection: Virus remains dormant within the host cell for long periods without causing symptoms (e.g., herpesviruses).

• Persistent Infection: Disease develops gradually over a long period and is often fatal (e.g., HIV/AIDS).

Viruses and Cancer

Some viruses can induce cancer by disrupting normal regulation of cell growth and division. They may carry oncogenes, activate host oncogenes, or interfere with tumor suppressor genes. Examples include Burkitt's lymphoma, Hodgkin's disease, Kaposi's sarcoma, and cervical cancer.

Culturing Viruses in the Laboratory

• Viruses require living cells for growth and cannot be cultured in standard media.

• Methods include using mature organisms, embryonated eggs, and cell cultures.

• Bacteriophages can be quantified using the plaque method, which counts plaque-forming units (PFU).

Identification of Viruses

• Electron microscopy

• Serological methods (antibody detection)

• Cytopathic effects in cell cultures

• Molecular techniques (RFLP, PCR)

Plant Viruses and Viroids

• Plant viruses cause symptoms such as color changes, deformed growth, wilting, and stunted growth.

• Infection often requires cell wall damage, which can be caused by parasites, wounds, or insect feeding.

• Viroids are infectious, naked, circular ssRNA molecules that do not code for proteins and are pathogenic in plants.

Prions

• Prions are infectious proteins that cause neurodegenerative diseases known as spongiform encephalopathies (e.g., mad cow disease, Creutzfeldt-Jakob disease, scrapie, kuru).

• They convert normal proteins into abnormal, infectious forms.

• Prions are resistant to standard inactivation methods and there is no effective treatment.

Comparison of Viruses, Viroids, Prions, and Bacterial Cells

Property	Viruses	Viroids	Prions	Bacterial Cells
Genetic Material	DNA or RNA	RNA	None	DNA
Protein Coat	Yes	No	No	Yes (cell membrane and wall)
Cellular Structure	No	No	No	Yes
Replication	Host cell machinery	Host cell machinery	Converts host proteins	Binary fission
Diseases	Animals, plants, bacteria	Plants	Animals	Animals, plants