Viruses: Structure, Replication, and Impact

Viruses: An Introduction

What Are Viruses?

Viruses are infectious agents responsible for diseases such as smallpox, influenza, Ebola, common cold, rabies, chickenpox, warts, AIDS, and H1N1. They are unique biological entities that challenge the definition of life due to their dependence on host cells for replication.

• Not considered living organisms by most biologists because they cannot reproduce or carry out metabolism independently.

• Composed of genetic material (DNA or RNA) enclosed in a protein coat.

• Obligate parasites: must infect a host cell to reproduce.

Are Viruses Alive?

The question of whether viruses are alive is debated. They lack cellular structure and metabolic machinery, but they can evolve and reproduce within host cells.

• Non-living by traditional criteria: no independent metabolism, cannot reproduce without a host.

• Living-like in their ability to evolve and adapt.

Naming and Classification of Viruses

Naming of Viruses

Viruses are named based on various conventions:

• By disease or symptoms: e.g., rhinovirus causes the common cold.

• By location: e.g., Ebola virus named after the Ebola River in Africa.

• By surface proteins: e.g., H1N1 influenza virus, where H and N refer to hemagglutinin and neuraminidase proteins.

• By letters and numbers: e.g., HIV, HPV.

Classification by Genetic Material

• DNA viruses: Contain DNA as genetic material.

• RNA viruses: Contain RNA as genetic material.

• Can be single-stranded or double-stranded.

Structure of Viruses

Basic Components

• Genetic Material: DNA or RNA, single or double stranded.

• Capsid: Protein coat made of subunits called capsomeres; determines the virus's shape.

• Envelope (in some viruses): Membranous layer derived from host cell membrane, containing both viral and host molecules.

• Spikes: Glycoproteins that aid in host cell attachment.

Shapes and Sizes

• Spherical (e.g., influenza virus)

• Icosahedral (20-faced, e.g., adenovirus)

• Rod-shaped (e.g., tobacco mosaic virus)

• Complex (e.g., bacteriophages with tails and fibers)

Virus Size Comparison Table

Virus	Dimensions (nm)
Bacteriophage M13	800 x 10
Vaccinia virus	300 x 200
Bacteriophage T4	225 x 100
Tobacco mosaic virus	250 x 18
Adenovirus	90
Poliovirus	30

Additional info: Human red blood cell diameter is about 10,000 nm; plasma membrane width is about 30 nm.

Common Features of All Viruses

1. Genetic Information: DNA or RNA, single or double stranded.

2. Protein Coat (Capsid): Varies in size and shape; determines the virus's form.

Specialized Virus Structures

Bacteriophages (Phages)

Bacteriophages are viruses that infect bacteria and have the most complex capsids.

• Capsid head: Encloses DNA.

• Tail piece: Attaches to host and injects DNA.

Host Range

The host range is the spectrum of host cells a virus can infect.

• Determined by specific attachment molecules (receptors) on host cells.

• Some viruses infect multiple species; others are highly specific.

• Reservoirs: Organisms that carry and transmit viruses without symptoms.

Viral Replication

General Steps

1. Entry and uncoating

2. Replication of viral genome

3. Transcription and manufacture of capsid proteins

4. Self-assembly and exit from host cell

Major Replication Strategies

• Lytic Cycle: Virus replicates immediately, causing host cell to lyse and release new viruses.

• Lysogenic Cycle: Viral DNA integrates into host genome as a prophage, replicates with host DNA, remains latent until triggered to become lytic.

Lytic vs. Lysogenic Cycle Table

Feature	Lytic Cycle	Lysogenic Cycle
Replication Speed	Immediate	Latent
Host Cell Fate	Lysis (death)	Cell survives, virus dormant
Viral DNA	Does not integrate	Integrates as prophage
Trigger	None needed	Environmental signal can activate lytic phase

Defenses Against Viruses

Bacterial Defenses

• Bacteria have mechanisms such as restriction enzymes and CRISPR systems to defend against phage infection.

Viruses as Pathogens

Viruses cause diseases in animals, plants, and humans. They can enter hosts via inhalation, ingestion, blood transfusion, or sexual contact. The death of infected cells leads to disease symptoms.

Combating Viruses

• Antibiotics: Ineffective against viruses.

• Antiviral drugs: Can treat but not cure viral infections.

• Vaccination: Most effective prevention method; uses nonpathogenic derivatives to stimulate immune response.

Emerging Viruses

Emerging viruses are those that suddenly become apparent, often due to mutation, spread from isolated populations, or transmission from animal reservoirs.

• Mutation: Especially common in RNA viruses.

• Spread from isolated populations: Disease may go unnoticed before spreading widely.

• Animal reservoirs: Many new human diseases originate from animals.

• Epidemic: Rapid spread in a population.

• Pandemic: Global spread across countries and continents.

Viral Diseases in Plants

• Over 2,000 types known; major impact on agriculture.

• Horizontal transmission: Infection from external source.

• Vertical transmission: Inheritance from parent plant.

• Symptoms: Bleached, brown, or blotchy leaves and fruit; abnormal growth.

Viroids and Prions

Viroids

• Infectious RNA molecules that attack plants.

• Cause stunted growth and abnormal development.

Prions

• Infectious proteins causing neurodegenerative diseases.

• Can convert normal proteins into abnormal, disease-causing forms.

• Resistant to heat, radiation, and chemicals.

• Examples: Mad cow disease, Kuru, Creutzfeldt-Jakob disease.

Viruses and Cancer

• Some viruses can initiate cancer by causing cell changes.

• Examples: Hepatitis B & C (liver cancer), HPV (cervical and other cancers), Epstein-Barr virus (lymphomas), HIV (increases risk for several cancers).

• Transmission: Sexual contact, blood, needle sharing, mother to fetus.

Key Terms and Concepts

• Capsid: Protein shell enclosing viral genome.

• Capsomere: Subunit of capsid.

• Envelope: Membranous layer derived from host cell.

• Prophage: Viral DNA integrated into host genome.

• Reservoir: Organism that harbors virus without symptoms.

• Viroid: Infectious RNA molecule in plants.

• Prion: Infectious protein causing neurodegenerative disease.

Important Equations and Concepts

• Virus Replication Equation: $\text{Virus} + \text{Host Cell} \rightarrow \text{Replication of Viral Genome} \rightarrow \text{Assembly of New Viruses}$

• Capsid Structure: $\text{Capsid} = \sum_{i=1}^{n} \text{Capsomeres}_i$

Summary Table: Virus Features

Feature	Description
Genetic Material	DNA or RNA, single or double stranded
Capsid	Protein coat, determines shape
Envelope	Present in some viruses, derived from host
Host Range	Specific to certain cells/species
Replication	Lytic or lysogenic cycle
Diseases	Wide range in animals, plants, humans

Additional info: This guide covers core concepts from General Biology Chapter 19 (Viruses), including structure, replication, host range, and impact on health and disease.