Viruses, Prions, and Viroids: Structure, Classification, and Life Cycles

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Characterizing and Classifying Viruses, Viroids, and Prions

Introduction to Viruses

Viruses are acellular infectious agents that require a host cell to replicate. They are distinct from living cells and exhibit unique structural and functional characteristics. Understanding their anatomy, classification, and replication cycles is essential for microbiology students.

Viruses infect a wide range of organisms, including plants, animals, and bacteria.
They are obligate parasites, meaning they cannot reproduce or carry out metabolic activities outside a host cell.
Viruses are generally considered non-living because they lack independent metabolic activity and cellular structure.
Examples of plant viruses include Tobacco mosaic virus, which historically impacted tobacco crops.

Are Viruses Living?

The debate over whether viruses are alive centers on their ability to reproduce and evolve, but their lack of metabolism and cellular structure outside a host cell leads to their classification as non-living.

Viruses are inert outside host cells.
They do not perform metabolic activities independently.
Cells contain both DNA and RNA; viruses contain either DNA or RNA, never both.

Viral Anatomy and Structure

Viruses have a simple structure, typically composed of four main components. Their size and shape are determined by their capsid and genome.

Nucleic Acid: Either DNA or RNA, single or double stranded, linear, circular, or fragmented.
Capsid: Protein shell made of subunits called capsomeres; determines the shape and protects the genome.
Envelope: Some viruses have a lipid bilayer envelope derived from the host cell membrane, decorated with viral proteins (spikes).
Spikes: Glycoprotein projections that facilitate host recognition and attachment; important for immune evasion.

Key Terms:

Virion: The extracellular form of a virus, outside the host cell.
Virus: The intracellular form, inside the host cell.

Viral Size and Shape

Viruses are much smaller than cells and often cannot be seen with light microscopes. Their shapes include helical, polyhedral, and spherical forms.

Size ranges from less than 30 nm (enteroviruses) to over 300 nm (megaviruses).
Shapes include helical (tube-like), polyhedral (icosahedral), and spherical (often enveloped).
Bacteriophages (viruses that infect bacteria) often have complex structures with heads and tails.

Viral Host Range and Specificity

The host range of a virus is determined by its ability to recognize and bind to specific receptors on host cells. This can be species-specific or cell-type specific.

Host range is defined by the spectrum of hosts a virus can infect.
Specificity can be down to cell type (e.g., SARS-CoV-2 infects cells expressing ACE2 receptors).
Some viruses infect only certain tissues (e.g., skin, neurons, salivary glands).

Viral Replication Cycles

Lytic Cycle

The lytic cycle is a viral replication process that results in the destruction (lysis) of the host cell and the release of new virions.

Attachment: Virus binds to specific receptors on the host cell.
Penetration: Viral genome is injected into the host cell.
Eclipse Period: Viral genes are expressed; host DNA is degraded, viral genome is replicated, and viral proteins are synthesized.
Assembly: New virions are assembled from replicated genomes and synthesized proteins.
Lysis: Host cell is lysed, releasing hundreds to thousands of new virions.

Key Enzymes:

Lysozyme: Enzyme used by bacteriophages to degrade bacterial cell walls for genome injection and cell lysis.
DNAse: Degrades host DNA to prevent host defense and provide nucleotides for viral replication.
DNA Polymerase: Replicates viral genome, often specialized to handle modified nucleotides.

Example: Bacteriophage T4 infects E. coli by binding to OMPC protein, injecting its genome, and using lysozyme to penetrate cell walls.

Lysogenic Cycle

The lysogenic cycle allows the viral genome to integrate into the host genome or persist as a plasmid, replicating along with the host cell without immediate destruction.

Integration: Viral genome integrates into host chromosome or attaches as a plasmid.
Replication: Viral genome is replicated with host cell division, remaining dormant.
Immunity: Infected cells become immune to further infection by similar viruses due to expression of suppressor genes.
Induction: Under certain conditions, the viral genome can excise and re-enter the lytic cycle, leading to cell lysis and virion release.

Example: Varicella-zoster virus (chickenpox) can become latent in neurons and later reactivate as shingles.

Memory Aid: Lysogenic has an 'O' (for circular chromosome integration).

Comparison: Lytic vs. Lysogenic Cycles

Feature	Lytic Cycle	Lysogenic Cycle
Host Cell Fate	Lysis and death	Survives, viral genome persists
Viral Genome	Replicates independently	Integrates or attaches to host genome
Virion Release	Rapid, large burst	Slow, few virions released
Immunity to Superinfection	No	Yes (due to suppressor genes)

Viral Classification

Major Classes of Viruses

Viruses are classified based on their nucleic acid type, strandedness, and replication strategy.

DNA Viruses: Genome is DNA; can be single or double stranded.
RNA Viruses: Genome is RNA; can be single or double stranded.
Retroviruses: Single-stranded RNA viruses that use reverse transcriptase to synthesize DNA from RNA.
DNA Viruses with RNA Intermediates: Double-stranded DNA viruses that use RNA intermediates during replication.

Key Enzyme: Reverse Transcriptase (in retroviruses) catalyzes the reaction:

Summary Table: Viral Classification

Class	Genome Type	Replication Strategy	Examples
DNA Viruses	Single or Double Stranded DNA	DNA replication	Adenoviruses, Herpesviruses
RNA Viruses	Single or Double Stranded RNA	RNA replication	Enteroviruses, Influenza virus
Retroviruses	Single Stranded RNA	Reverse transcription (RNA to DNA)	HIV
DNA Viruses with RNA Intermediates	Double Stranded DNA	RNA intermediate in replication	Hepatitis B virus

Additional info:

Some viruses maintain their genome as plasmids during lysogeny to avoid integration errors and reduce genome size requirements.
Viral host range is determined by the compatibility of viral spikes/capsid proteins with host cell receptors.
Lysozyme activity is more pronounced in bacteriophages infecting gram-positive bacteria due to thicker cell walls.
Restriction enzymes in bacteria provide defense against viral infection; viruses counteract with modified nucleotides (e.g., methylated cytosine).