General Virus Characteristics

Overview

Viruses are unique infectious agents classified as nonliving microbes due to their inability to carry out metabolic processes independently. They infect a wide range of hosts, including prokaryotic and eukaryotic cells, and exhibit diverse structural and genomic features.

• Nonliving Microbes: Viruses lack cellular structure and metabolism.

• Host Range: Viruses infect bacteria, animals, and plants.

• Structural Diversity: Includes capsids, envelopes, spikes, and various genome types.

• Genomic Variation: DNA and RNA viruses, single- or double-stranded genomes.

• Antigenic Drift and Shift: Genetic changes in viral antigens can lead to new outbreaks.

Viruses as Nonliving Pathogens

Key Features

Viruses are submicroscopic infectious agents studied in the field of virology.

• Over 5,000 mammal-infecting viral species described; many remain uncharacterized.

• Viruses are obligate intracellular pathogens, requiring host cells for replication.

• Extremely small (20–400 nm).

Virus Structure and Genomic Features

Virion

A virion is a single, infectious virus particle.

• Contains a protein shell called the capsid and genetic material (DNA or RNA).

• Capsid is made of capsomere subunits and protects the genome.

Capsid Types

• Helical capsids: Hollow tube structure.

• Icosahedral capsids: Three-dimensional polygonal structure.

• Complex capsids: Deviations from helical or icosahedral forms.

Viral Envelopes

• Enveloped viruses: Have a lipid-based envelope surrounding the capsid.

• Naked viruses: Lack an envelope; may arise from lysing the host cell.

Viral Spikes (Peplomers)

• Spikes are glycoprotein extensions that help viruses attach and gain entry to host cells.

• Influenza virus spikes: Hemagglutinin (HA) and Neuraminidase (NA).

Viral Genomes

• Most viruses have fewer than 300 genes.

• Genomes can be single- or double-stranded, DNA or RNA, circular or linear.

Virus Genome Types and Replication

DNA Viruses

• Double-stranded DNA (dsDNA): Transcribed using host RNA polymerases.

• Single-stranded DNA (ssDNA): Converted to double-stranded form before transcription.

RNA Viruses

RNA viruses have different modes of making mRNA for protein synthesis.

• Single-stranded positive RNA (+ssRNA): Functions as mRNA.

• Single-stranded negative RNA (−ssRNA): Complementary to mRNA; requires RNA-dependent RNA polymerase.

• Double-stranded RNA (dsRNA): Requires RNA-dependent RNA polymerase.

• Retroviruses: RNA genome is reverse transcribed into DNA.

Viral Genome Change Over Time

Genetic Variation

Viruses exhibit rapid genomic changes due to high mutation rates and recombination.

• Quick replication cycles and large quantities of virions produced.

• RNA polymerases lack proofreading, leading to mutations.

• Genetic changes can result in attenuated strains (less virulent forms).

Antigenic Drift and Antigenic Shift

Definitions

• Antigenic drift: Minor genetic changes in viral antigens (e.g., influenza HA and NA genes).

• Antigenic shift: Major genetic reassortment causing new viral strains and pandemics.

Classifying and Naming Viruses

Criteria

Viruses are classified by the International Committee on Taxonomy of Viruses (ICTV) using:

• Type of nucleic acid (DNA or RNA)

• Capsid structure

• Presence or absence of envelope

• Genome architecture (ssDNA, ssRNA, etc.)

Host Range and Tropism

Definitions

• Host range: Species a virus can infect.

• Tropism: Specificity for tissues or cell types.

• Some viruses have broad tropism; others are highly specific (e.g., measles virus infects only humans).

Virus Sizes

Range

• Rhinoviruses and polioviruses: ~30 nm diameter.

• Ebola and Pandoraviruses: up to 1,000–1,500 nm length.

Virus Taxonomy

Standardized Ranks

Viruses are not assigned to domains or kingdoms. The highest taxon is phylum, followed by:

• Order

• Family (and occasionally subfamily)

• Genus

• Species

Introduction to Viral Replication Pathways

General Principles

Viruses hijack host cell machinery to multiply, using the host's energy, enzymes, and organelles.

• Replication pathways differ between bacteriophages and animal viruses.

• Enveloped and naked viruses have distinct entry and exit mechanisms.

Bacteriophage Replication

Lytic Replication Pathway

• Virus infects bacterial cell, immediately builds new virions, and kills the host cell upon release.

• Steps: Attachment, Penetration, Replication, Assembly, Release.

Lysogenic Replication Pathway

• Phage genome integrates into host genome as a prophage.

• Prophage may confer new properties (e.g., toxin production).

• Examples: Corynebacterium diphtheriae, Clostridium botulinum.

Generalized Animal Virus Replication

Six Steps

1. Attachment: Virus binds to host cell membrane.

2. Penetration (Entry): Virus enters cell via endocytosis or membrane fusion.

3. Uncoating: Capsid is digested, releasing genome.

4. Replication (Synthesis): Genome is replicated, viral proteins are made.

5. Assembly: New virions are formed.

6. Release: Virions exit the cell by lysis or budding.

Persistent Infections

Types

• Acute infections: Immediate production of new virions.

• Chronic infections: Continuous release over months or years (e.g., HIV).

• Latent infections: Flare-ups with periods of dormancy (e.g., herpesviruses).

Persistent Infections Leading to Cancer

• Oncogenic viruses: Cause cancer by stimulating uncontrolled cell division.

• Examples: Human papillomavirus (HPV), Human T-lymphotropic virus (HTLV).

Laboratory Methods for Growing Viruses

Plaque Assays

• Bacteriophages grown on bacterial plates; clear zones (plaques) indicate lysis.

• Plaque-forming units (PFUs): Quantify bacteriophages in a sample.

Growing Animal Viruses

• Animal viruses require host cells; often grown in tissue culture or embryonated eggs.

Diagnostic Tests for Viruses

Purpose

• Detect viral proteins and genetic material for diagnosis.

• Ensure safety in medical procedures (e.g., transplants).

Summary Table: Virus Types and Features

Key Equations

• Viral titer calculation:

Additional info:

• Some context and definitions were expanded for clarity and completeness.

• Examples and applications were added to illustrate key concepts.