Viruses, Viroids, and Prions: Structure, Classification, and Replication

Viruses, Viroids, and Prions

Introduction to Viruses

Viruses are acellular infectious agents that require living host cells to multiply. They are distinct from bacteria and other microorganisms due to their unique structure and replication mechanisms.

• Obligate intracellular parasites: Viruses cannot reproduce or carry out metabolic processes outside a host cell.

• Genetic material: Contain either DNA or RNA, but never both.

• Protein coat: The nucleic acid is surrounded by a protein coat called a capsid.

• No ribosomes or ATP-generating mechanisms: Viruses lack the cellular machinery for protein synthesis and energy production.

• Extracellular and intracellular states: Viruses exist as inert particles outside cells but become active within host cells.

Are viruses alive? Viruses are considered non-living outside host cells but exhibit characteristics of life (such as replication and evolution) when inside a host cell.

Comparison: Viruses vs. Bacteria

Viruses and bacteria differ in several fundamental ways, as summarized in the table below:

Characteristic	Bacteria	Viruses
Intracellular Parasite	No	Yes
Plasma Membrane	Yes	No
Binary Fission	Yes	No
Pass through Bacteriological Filters	No	Yes
DNA and RNA	Yes (both)	No (one or the other)
ATP-generating Metabolism	Yes	No
Ribosomes	Yes	No
Sensitive to Antibiotics	Yes	No
Sensitive to Interferon	No	Yes

Size and Morphology of Viruses

Viruses are much smaller than bacteria and eukaryotic cells, typically ranging from 20 to 1000 nanometers in length. Their small size contributed to their discovery before the invention of the electron microscope.

• Helical viruses: Hollow, cylindrical capsid (e.g., Tobacco mosaic virus).

• Polyhedral viruses: Many-sided, often icosahedral (e.g., Adenovirus).

• Enveloped viruses: Surrounded by a lipid, protein, and carbohydrate envelope (e.g., Influenza virus).

• Complex viruses: Complicated structures (e.g., Bacteriophage T4).

Viral Structure

The basic structure of a virus includes:

• Nucleic acid: DNA or RNA, single- or double-stranded, linear or circular.

• Capsid: Protein coat made of subunits called capsomeres, providing protection and aiding in attachment to host cells.

• Envelope (in some viruses): Acquired from the host cell membrane during replication or release; contains viral proteins and glycoprotein spikes.

• Spikes: Carbohydrate-protein complexes that project from the envelope and are involved in host cell recognition and attachment.

Hosts and Host Range

Viruses infect a wide range of hosts, including animals, plants, bacteria (bacteriophages), and fungi. The host range is determined by the virus's ability to attach to specific receptor sites on the host cell surface and the presence of necessary cellular factors for viral replication.

• Specificity: Some viruses infect only a particular cell type in a specific host, while others (generalists) can infect many cell types across different hosts.

• Attachment: Mediated by interactions between viral surface proteins (or glycoproteins) and host cell receptors.

Viral Taxonomy

Viruses are classified based on their genetic material, structure, host range, and replication strategy.

• Genus names: End in -virus

• Family names: End in -viridae

• Order names: End in -ales

• Viral species: Group of viruses sharing the same genetic information and ecological niche (host)

Viral Replication Cycles

Lytic Cycle (Bacteriophages)

The lytic cycle results in the destruction of the host cell and the release of new virions. The stages are:

1. Attachment: Phage attaches to host cell.

2. Penetration: Phage injects its DNA into the host.

3. Biosynthesis: Host machinery synthesizes viral components.

4. Maturation: Assembly of new phage particles.

5. Release: Host cell lyses, releasing new phages.

Lysogenic Cycle (Temperate Phages)

In the lysogenic cycle, the phage DNA integrates into the host genome as a prophage and replicates along with the host cell without causing immediate lysis. The prophage may later enter the lytic cycle.

• Phage conversion: Host cell exhibits new properties due to prophage genes.

• Specialized transduction: Specific bacterial genes are transferred to another bacterium via a phage.

Animal Virus Replication

Animal viruses differ from bacteriophages in their entry, uncoating, and release mechanisms:

• Attachment: Via glycoprotein spikes or other molecules to host cell membrane.

• Entry: By receptor-mediated endocytosis or fusion.

• Uncoating: Removal of capsid by viral or host enzymes.

• Biosynthesis: DNA viruses often replicate in the nucleus; RNA viruses in the cytoplasm.

• Maturation and Release: Enveloped viruses bud from the host cell; nonenveloped viruses may cause cell lysis.

Genetic Material and Classification

Viruses are classified by the type and structure of their nucleic acid:

• DNA viruses: Double- or single-stranded, linear or circular.

• RNA viruses: Single- or double-stranded, positive-sense (+) or negative-sense (−).

• Retroviruses: Single-stranded RNA viruses that use reverse transcriptase to synthesize DNA from RNA.

Examples of Virus Families Affecting Humans

Family	Genome	Envelope	Example	Disease
Adenoviridae	dsDNA	No	Adenovirus	Respiratory infections
Herpesviridae	dsDNA	Yes	Herpes simplex virus	Cold sores, genital herpes
Picornaviridae	ssRNA (+)	No	Poliovirus, Rhinovirus	Polio, common cold
Retroviridae	ssRNA (+)	Yes	HIV	AIDS

Viroids and Prions

• Viroids: Small, circular RNA molecules that infect plants, lacking a protein coat (e.g., Potato spindle tuber viroid).

• Prions: Infectious proteins that cause neurodegenerative diseases in animals and humans (e.g., Creutzfeldt-Jakob disease, mad cow disease).

Summary Table: Virus, Viroid, and Prion Comparison

Agent	Genetic Material	Protein Coat	Host(s)	Example Disease
Virus	DNA or RNA	Yes	Animals, plants, bacteria	Influenza, HIV
Viroid	RNA	No	Plants	Potato spindle tuber disease
Prion	None	No	Animals, humans	Mad cow disease, CJD