6-Viruses: Structure, Classification, and Infection Cycles

Viruses: An Overview

What is a Virus?

Viruses are microscopic infectious agents that require a host cell to replicate. They are fundamentally different from living cells in both structure and function.

• Size: Typically 10–300 nm, much smaller than cells.

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

• Structure: Genetic material is wrapped in a protein coat (capsid); some viruses have a lipid envelope derived from the host cell membrane.

• Metabolism: No metabolism; cannot generate energy or synthesize proteins independently.

• Replication: Can only multiply within living cells of a host.

Are Viruses Living?

The status of viruses as living or non-living is debated. They lack many characteristics of living organisms.

Living Organisms	Viruses
Composed of one or more cells	Not cellular; composed of genetic material and protein
Organelles for specialized functions	No organelles
Energy flow (metabolism) occurs within cells	No metabolism; do not generate energy
Arise from pre-existing cells (self-replicate)	Cannot reproduce on their own; require host cell

Classification and Types of Viruses

Classification Criteria

Viruses are classified based on several features:

• Morphology (Shape): Helical, icosahedral, or complex

• Genome Material: DNA or RNA, single-stranded (SS) or double-stranded (DS), linear or circular

• Type of Replication: Lytic or lysogenic cycles

• Host: Animals, plants, bacteria (bacteriophages), archaea

• Type of Disease: Influenza, HIV, hepatitis, etc.

Examples of Viruses

• Ebola virus (Filovirus)

• Bacteriophage (infects bacteria)

• Adenovirus (causes respiratory infections)

• Coronavirus (causes COVID-19)

Viral Structure

Basic Components

All viruses share a basic structure:

• Nucleic Acid: DNA or RNA (genome)

• Capsid: Protein shell that encloses the genome; made of subunits called capsomeres

• Nucleocapsid: Genome + capsid protein

• Envelope (in some viruses): Lipid bilayer derived from host cell membrane, may contain glycoproteins

Types of Capsid Symmetry

• Helical: Capsomeres arranged in a spiral around the nucleic acid (e.g., Tobacco mosaic virus, Ebola virus)

• Icosahedral: Capsid forms a 20-faced polyhedron; allows for a closed shell with minimal subunits (e.g., Adenovirus)

• Complex: Neither purely helical nor icosahedral; may have additional structures like tails (e.g., Bacteriophage)

Enveloped Viruses

Some viruses acquire a lipid envelope from the host cell membrane during budding. The envelope is virus-specific and may contain viral glycoproteins.

• Envelope does not encode lipid synthetic machinery.

• Nucleocapsids inside the envelope may have helical or icosahedral symmetry.

Viral Genomes

Genome Types

Viral genomes vary in structure:

• DNA Viruses: Linear or circular, single-stranded (SS) or double-stranded (DS)

• RNA Viruses: Linear or circular, SS or DS

Examples:

• Parvovirus: SS DNA

• Herpesvirus: DS DNA

• TMV (Tobacco mosaic virus): SS RNA

• Reovirus: DS RNA

Information Encoded in Viral Genomes

• Protein synthesis (mainly in giant viruses)

• Replication of the viral genome

• Assembly and packaging of the genome

• Regulation and timing of the replication cycle

• Modulation of host defenses

• Spread to other cells and hosts

Information Not Encoded

• No genes for complete protein synthesis machinery

• No genes for membrane biosynthesis

• No classical centromeres or telomeres

Viral Infection Cycles

Lytic Cycle

The lytic cycle is a process by which viruses replicate and destroy the host cell.

1. Attachment: Virus binds to the host cell surface.

2. Penetration: Injection of viral DNA or RNA into the host cell.

3. Replication (Biosynthesis): Host cell machinery synthesizes viral proteins and nucleic acids.

4. Assembly (Maturation): New viral particles are assembled.

5. Release (Lysis): Host cell bursts, releasing new viruses.

One-Step Growth Curve

Describes the increase in virus particles after a single round of infection and lysis.

Lysogenic Cycle

Some viruses can integrate their genome into the host cell's chromosome, remaining dormant (latent) for extended periods.

• Viral genome incorporated into host chromosome as a prophage.

• Host cell replicates, passing viral genes to daughter cells.

• Activation signals (e.g., chemicals, radiation, stress) can trigger switch to lytic cycle.

Latency in Eukaryotes

• Chickenpox (Varicella zoster): Remains dormant in nervous tissue, can reactivate as shingles.

• Herpes viruses: Latent in nervous system; infections can last a lifetime (Herpes Simplex 1 and 2).

Emerging Viruses

Definition and Importance

Emerging viruses are causative agents of new or previously unrecognized infections. The term became popular in the 1990s, but such viruses have existed throughout history.

• Expanded host range and increased disease incidence.

• Transmission from wild or domesticated animals to humans (zoonosis).

• Cross-species infection may establish new viruses in populations (e.g., SARS-CoV-2, HIV).

• Some cross-species infections are not sustained (e.g., Ebola from bats to humans).

Factors Driving Emergence

• Human population growth and global change.

• Ecological and anthropogenic activities: urbanization, deforestation, climate change, global travel, agriculture, hunting, and animal trade.

Examples of Emerging Viruses

Virus	Family	Factors Leading to Emergence
Dengue virus	Flaviviridae	Urbanization, water storage, mosquito breeding
Ebola virus	Filoviridae	Human contact with natural host (bats)
Hantavirus	Bunyaviridae	Agriculture, human-rodent contact
Human immunodeficiency virus (HIV)	Retroviridae	Hunting, butchering of infected primates
Middle East respiratory syndrome (MERS)	Coronaviridae	Camel husbandry, contact with humans
Nipah virus	Paramyxoviridae	Fruit bats, natural reservoir, transmission to pigs and humans
West Nile virus	Flaviviridae	Mosquito transmission, global travel
Zika virus	Flaviviridae	Mosquito, global travel

Summary Table: Viruses, Bacteria, and Protozoa

Feature	Viruses	Bacteria	Protozoa
Size	10–300 nm	50–2000 nm	1–300 μm
Cell Type	Not cellular	Prokaryotic	Eukaryotic
Internal Structure	Protein coat, genetic material	Proteins, genetic material, fats	Organelles, internal structure
Metabolic Activity	Not metabolically active	Metabolically active	Metabolically active
Replication	Requires host cell	Self-replicating	Self-replicating
Examples	Influenza, HIV, hepatitis	E. coli, S. aureus	Amoeba, Cryptosporidium

Key Terms

• Capsid: Protein shell enclosing viral genome

• Capsomere: Subunit of capsid

• Nucleocapsid: Genome + capsid

• Envelope: Lipid bilayer from host cell

• Lytic cycle: Viral replication resulting in host cell lysis

• Lysogenic cycle: Viral genome integrates into host DNA, remains dormant

• Prophage: Integrated viral DNA in host genome

• Zoonosis: Transmission of virus from animals to humans

Important Equations and Concepts

• One-Step Growth Curve: Describes the burst size and timing of virus release from infected cells.

• Genome Classification: $\text{Viral Genomes} \rightarrow \begin{cases} \text{DNA} \rightarrow \begin{cases} \text{Linear} \\ \text{Circular} \end{cases} \\ \text{RNA} \rightarrow \begin{cases} \text{Linear} \\ \text{Circular} \end{cases} \end{cases}$

Additional info: These notes expand on the original slides by providing definitions, examples, and context for viral structure, classification, and infection cycles, suitable for college-level Cell Biology study.