chap 6A

General Virus Characteristics

Introduction

Viruses are unique infectious agents that differ fundamentally from cellular life forms such as prokaryotes and eukaryotes. This section explores their defining features, classification, and structural components, providing a foundation for understanding their role in microbiology.

• Viruses are classified as nonliving microbes because they lack cellular structure and metabolic processes.

• Virology is the scientific study of viruses.

• Viruses are submicroscopic (typically 20–400 nm), acellular, and obligate intracellular pathogens—they require a host cell to replicate.

• Over 5,000 mammal-infecting viral species have been described, with about 220 known to infect humans. However, it is estimated that at least 320,000 mammalian viruses remain uncharacterized.

Comparing Viruses, Prokaryotes, and Eukaryotes

Key Differences and Similarities

Viruses differ from prokaryotic and eukaryotic cells in several fundamental ways, including structure, replication, and genetic material.

Characteristic	Viruses	Prokaryotes	Eukaryotes
Cellular?	No	Yes	Yes
Considered alive?	No	Yes	Yes
Relative size	Generally smaller than prokaryotes and eukaryotes; pass through filters that retain bacteria	Larger than viruses but smaller than eukaryotes	Largest; usually visible under light microscope
Structure	Protein capsid coating and nucleic acid	Cell wall and plasma membrane (some have capsule)	Cells with nucleus and membrane-bound organelles
Replication	Host cell energy and machinery are required to replicate the virus	Binary fission (asexual)	Mitosis (asexual) and meiosis (sexual)
Genetic material	DNA or RNA	DNA	DNA

Genomic Features of Viruses

Host Range and Focus

Viruses can infect organisms across all domains of life. In microbiology, particular attention is given to those infecting bacteria and animals.

• Bacteriophages (phages): Viruses that infect bacteria.

• Animal viruses: Viruses that infect animals, including humans.

Virion

• A virion is a single, infectious virus particle.

• It consists of an exterior protective protein capsid and genetic material (either DNA or RNA).

Capsid Structures

Types and Functions

The capsid is a protein shell that encases and protects the viral genome. Its structure is crucial for the virus's ability to infect host cells.

• Capsid: Protein shell that packages and protects the genome; accounts for most of the virion's mass.

• Made of protein subunits called capsomeres.

• Most animal viruses have either helical or icosahedral capsids:

  • Helical capsids: Resemble a hollow tube.

  • Icosahedral capsids: Look like three-dimensional polygons.

  • Complex capsids: Deviations from helical or icosahedral structures, often seen in bacteriophages.

Bacteriophage Capsid Structure

• Bacteriophages typically have complex capsid structures, often with icosahedral symmetry.

• These structures may include additional components (e.g., tail fibers, baseplates) that facilitate genome injection into host cells.

Viral Envelopes

Enveloped vs. Naked Viruses

The presence or absence of a lipid envelope is a key feature in viral classification and affects how viruses interact with host cells.

• Enveloped viruses: Possess a lipid-based envelope surrounding the capsid, acquired by budding off the host cell and incorporating part of the cell membrane.

• Naked (nonenveloped) viruses: Lack an envelope and typically arise from lysing (bursting) the host cell.

• Animal viruses may be either enveloped or naked.

• Bacteriophages always lyse host cells and are therefore always naked.

Viral Spikes (Peplomers)

Structure and Function

Many viruses possess spike proteins (peplomers) that extend from the capsid or envelope, playing a critical role in host cell recognition and attachment.

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

• Spikes determine host specificity and are targets for immune recognition.

• Example: SARS-CoV-2 uses multiple spike proteins to bind to host cell receptors.

Influenza Virus Spikes

Hemagglutinin (HA) and Neuraminidase (NA)

Influenza viruses are notable for their rapidly mutating spike proteins, which impact infectivity and immune response.

• Hemagglutinin (HA): Facilitates viral entry into host cells.

• Neuraminidase (NA): Assists in viral release from host cells.

• Frequent mutations in HA and NA lead to antigenic drift and shift, contributing to influenza outbreaks and pandemics.

Summary Table: Key Features of Viruses

Feature	Description
Size	20–400 nm (submicroscopic)
Structure	Capsid (protein shell), may have envelope, spikes
Genetic Material	DNA or RNA (never both)
Replication	Obligate intracellular; uses host cell machinery
Host Range	Can infect all branches of life; specific host range determined by viral structure

Key Terms and Definitions

• Virion: A complete, infectious virus particle.

• Capsid: Protein shell encasing the viral genome.

• Capsomere: Protein subunit of the capsid.

• Envelope: Lipid membrane surrounding some viruses.

• Spike (Peplomer): Glycoprotein projection involved in host cell attachment.

• Bacteriophage: Virus that infects bacteria.

• Antigenic drift: Gradual mutation in viral antigens.

• Antigenic shift: Abrupt, major change in viral antigens, often leading to pandemics.

Additional info:

• Viruses are not assigned to domains or kingdoms in biological taxonomy; their highest taxonomic rank is the "realm."

• Virus classification is based on nucleic acid type, capsid symmetry, presence/absence of envelope, and genome architecture.