Viruses and Prions: Structure, Classification, and Key Features CHAPTER 6A

Viruses and Prions

Introduction

This section introduces the fundamental characteristics of viruses and prions, focusing on their structure, classification, and significance in microbiology. Viruses are unique infectious agents that differ significantly from prokaryotic and eukaryotic cells, and understanding their properties is essential for studying infectious diseases.

General Virus Characteristics

Definition and Nature of Viruses

• Viruses are submicroscopic, infectious agents that are acellular and require a host cell to replicate.

• Virology is the scientific study of viruses.

• 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.

• Viruses are obligate intracellular pathogens, meaning they can only reproduce inside living host cells.

• They are extremely small, typically ranging from 20–400 nm in size.

Comparison: Viruses, Prokaryotes, and Eukaryotes

Viruses differ fundamentally from prokaryotic and eukaryotic cells in structure, replication, and metabolism.

Characteristic	Viruses	Prokaryotes	Eukaryotes
Cellular?	No	Yes	Yes
Considered alive?	No	Yes	Yes
Relative size	Generally smaller than prokaryotes; most require electron microscopy to be seen	Larger than viruses but smaller than eukaryotes; usually visible with light microscopy	Largest; visible under light microscopy
Pass through a filter designed to trap cells?	Yes	No	No
Structure	Protein coat (capsid) and nucleic acid	Cells with nucleoid and cell wall (most)	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)
Enzyme metabolism?	No	Yes	Yes
Genetic composition	DNA or RNA	DNA	DNA

Table purpose: Comparison of fundamental properties of viruses, prokaryotes, and eukaryotes.

Genomic Features of Viruses

Host Range and Focus

• Viruses can infect every branch in the tree of life.

• Bacteriophages (phages) infect bacteria.

• Animal viruses infect animals and humans.

Virion Structure

• A virion is a single, infectious virus particle.

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

Viral Structure

Capsid

• The capsid is a protein shell that packages and protects the viral genome.

• It accounts for the bulk of a virion’s mass and is made of protein subunits called capsomeres.

• Most animal viruses have either helical or icosahedral capsids:

  • Helical capsids resemble a hollow tube.

  • Icosahedral capsids are shaped like three-dimensional polygons.

  • Capsids that deviate from these two forms are classified as complex capsids.

• Bacteriophages often have complex capsid structures, usually with icosahedral symmetry and additional components (e.g., tail fibers) that enable them to inject their genome into target cells.

Viral Envelopes

• Enveloped viruses possess a lipid-based envelope that surrounds the capsid.

  • These envelopes arise from budding off the host cell, taking a portion of the cell membrane with them.

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

• Animal viruses may be either enveloped or naked.

• Bacteriophages lyse host cells and are therefore always naked.

Viral Spikes (Peplomers)

• Many viruses have spikes (peplomers), which are protein extensions that protrude from the viral capsid or envelope.

• Spikes are glycoproteins that help viruses attach and gain entry to host cells by binding to specific receptors.

• Example: SARS-CoV-2 uses multiple receptors to enter host cells.

Influenza Virus Spikes

• Influenza viruses frequently mutate, leading to small changes in spike proteins.

• Key influenza A spikes:

  • Hemagglutinin (HA)

  • Neuraminidase (NA)

• These spikes determine how the virus interacts with host cells and how the immune system recognizes the virus.

Summary Table: Key Features of Viruses

Feature	Description
Size	20–400 nm (submicroscopic)
Structure	Capsid (protein shell), may have envelope, contains DNA or RNA
Replication	Obligate intracellular; requires host cell machinery
Host Range	Can infect all forms of life (bacteria, animals, plants, etc.)
Spikes	Protein/glycoprotein structures for host cell attachment

Example: Influenza Virus

• Enveloped virus with helical capsid.

• Contains RNA genome.

• Has HA and NA spikes that are important for host cell entry and immune recognition.

Additional info: The notes above are expanded with academic context to clarify the structure and classification of viruses, as well as their comparison to cellular life forms. The tables are reconstructed to summarize key differences and features for exam preparation.