Comprehensive Study Notes on General Virus Characteristics and Replication (Microbiology)

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

Virus Type	Genome	Envelope	Host Range	Replication Pathway
Bacteriophage	DNA or RNA	Naked	Bacteria	Lytic/Lysogenic
Animal Virus	DNA or RNA	Enveloped or Naked	Animals	Generalized (6 steps)
Plant Virus	RNA (mostly)	Naked	Plants	Varied

Key Equations

• Viral titer calculation: $\text{Viral titer} = \frac{\text{Number of plaques}}{\text{Volume of sample plated}}$

Additional info:

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

• Examples and applications were added to illustrate key concepts.