BackViruses and Prions: Structure, Classification, and Replication
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General Virus Characteristics
Viruses as Nonliving Pathogens
Viruses are unique infectious agents that differ fundamentally from cellular life forms. They are considered nonliving because they lack the cellular machinery necessary for independent metabolism and reproduction.
Virology: The scientific study of viruses.
Viruses are extremely small (20-400 nm), acellular, and nonliving.
They infect every branch of life, including animals, plants, fungi, and bacteria.
Viruses cannot synthesize molecules or generate energy on their own; they must reproduce inside a host cell.
Obligate intracellular pathogens: Viruses require a host cell for replication.
Additional info: Viruses are not classified as living organisms because they do not possess the characteristics of life, such as cellular structure, metabolism, and independent reproduction.
Virus Diversity and Host Range
Viruses exhibit remarkable diversity in their host range and genetic makeup.
Over 5,000 mammal-infecting viral species have been described; about 220 infect humans.
It is estimated that at least 320,000 mammalian viruses remain uncharacterized.
Many viruses that infect humans are harbored by other animals, highlighting the importance of zoonotic transmission.
Comparing Viruses, Prokaryotes, and Eukaryotes
Key Differences
Viruses differ from prokaryotic and eukaryotic cells in several fundamental ways. The following table summarizes these differences:
Characteristic | Viruses | Prokaryotes | Eukaryotes |
|---|---|---|---|
Cellular? | No | Yes | Yes |
Considered alive? | No | Yes | Yes |
Size | Generally smaller than prokaryotes; most require electron microscopy to see | Most bigger than viruses and smaller than eukaryotes; usually seen with light microscopy | Usually bigger than prokaryotes and often visible with light microscopy |
Cell membrane? | No | No (except for some very small filterable bacteria) | Yes |
Genetic material | Protein capsid coating and nucleic acid | Cells without nuclei or other membrane-bound organelles | Cells with nuclei and membrane-bound organelles |
Replication | Host cell energy and machinery are hijacked to replicate the virus | Binary fission (asexual) | Mitosis (asexual) and meiosis (sexual) |
Metabolism? | No | Yes | Yes |
Genetic composition | DNA or RNA | DNA | DNA |
Viral Structure and Genomic Features
Virion Structure
The virion is the complete, infectious virus particle. It consists of the viral genome (either DNA or RNA) encased in a protein shell called the capsid. Some viruses also possess a lipid envelope derived from the host cell membrane.
Capsid: Protein shell that packages and protects the genome; made of capsomere subunits.
Envelope: Some viruses have a membrane envelope; those lacking it are termed naked viruses.
Capsid Shapes
Capsids can have different shapes, which are important for virus classification and function.
Helical capsids: Appear as hollow tubes (e.g., tobacco mosaic virus).
Icosahedral capsids: Appear as three-dimensional polygons (e.g., adenovirus).
Complex capsids: Variations from the above, often seen in bacteriophages with additional structures for genome injection.
Additional info: The capsid shape influences how the virus attaches to and enters host cells.
