Virus Structure
Viruses are non-cellular infectious particles that are capable of causing disease within a host organism
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They are obligate intracellular parasites that hijack a cell’s molecular machinery and energy resources
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By commandeering a host cell’s resources, they cause the disruption of normal homeostatic processes
Viruses are not considered to be living because they cannot independently perform all the functions of life
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They lack metabolism, cannot replicate autonomously, are unable to grow and do not maintain homeostasis
Viruses are very diverse in structure, however all share some basic features:
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Protein coat – Viruses are enclosed within a protein shell called a capsid
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Internal components – Viruses do not contain any cytoplasm and have few or no enzymes
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Genetic material – Viruses contain a nucleic acid core consisting of either DNA or RNA
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Size – Viruses contain few molecules and so have a small, fixed size (they do not grow)
Virus Structure Diagram
Types of Viruses
Because viruses are non-cellular, they possess none of the characteristic features that are used to classify living organisms
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However, viruses are highly diverse and display significant variation in their genetic composition, shape and structure
Viruses can be differentiated according to a number of properties:
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The genetic material may be DNA or RNA, which may be either double-stranded or single-stranded
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The viral genome may be comprised of either circular or linear molecules of nucleic acid
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Viruses can exist in a range of sizes (although all are small and typically range from 20 – 300 nm)
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The structure of the protein shell can vary (capsids can be helical, polyhedral, spherical or complex)
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Viruses may possess distinct attachment proteins according to the type of host cell they infect
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Some viruses are enveloped within a lipid membrane, while other viruses are not enveloped
Examples of viruses with different characteristics include:
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Bacteriophage lambda – A non-enveloped virus with a complex structure composed of ds-DNA (linear)
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Coronavirus – An enveloped virus containing ss-RNA (linear) within a spherical capsid (cause of Covid-19)
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HIV – An enveloped retrovirus consisting of ss-RNA within an icosahedral capsid (infects helper T cells)
Virus Structures
Helical
Polyhedral
Spherical
Complex