A Virion Contains a Genome and Protective Structures
A virion cannot reproduce on its own. It has no ribosomes and cannot generate energy, so the host cell copies the viral genome and builds the protein coat around it.
Parts of a Virion
When someone has the flu, we often say they "caught a virus." Viruses are acellular particles, which means they are not organized as complete cells. Bacteria, by contrast, are cells.
One complete virus particle is called a virion. A virion carries a genome, genetic material made of DNA or RNA. The genome is enclosed by a capsid, a protein coat built from smaller subunits called capsomeres. Some viruses also have a lipid envelope outside the capsid, a membrane-like layer taken from the host cell.
Virus classification compares genome type, capsid structure, the presence of an envelope, gene-expression strategy, host range, and sequence relationships.
Virion Parts Have Different Functions
Viruses can be compared by their composition, structure, classification, and effects on hosts. The table below compares the major parts found in many viruses.
| Virion part | Function | Variation among viruses |
|---|---|---|
| Genome | Carries genetic instructions | It can be DNA or RNA, depending on the virus type |
| Capsid | Protects the genome | Protein units join to form this protective coat |
| Lipid envelope | Carries viral proteins in a lipid layer | Some viruses have it, but not all viruses do |
| Surface protein | Binds compatible molecules on a target cell | Its form and target differ among virus types |
A virion carries genetic information and protective structures. It depends on a host cell for protein production, energy, and other processes needed for replication.
The capsid is a protein coat that protects the viral genome outside a host cell.
- What to examine
- The faceted model shows many capsomeres joined into one capsid.
- Biological meaning
- Without a capsid, the viral genome would be exposed before reaching a suitable cell.
The next model shows the surface of SARS-CoV-2 in more detail than the general diagram above. Cryo-electron tomography of intact virions found a lipid bilayer with spike trimers protruding from it. The spikes bind the ACE2 receptor and mediate entry into suitable target cells. This model represents one enveloped virus. Other viruses can have different shapes and parts.
SARS-CoV-2 appears as an enveloped virion with a rough surface and many protein projections.
- What to examine
- The model shows a rounded envelope with protein projections distributed across its surface.
- Biological meaning
- SARS-CoV-2 spike proteins can bind ACE2 on suitable cells and mediate viral entry into the cell.
Viral Replication Uses a Host Cell
Living cells usually have a cell membrane, cytoplasm, ribosomes, and metabolic reactions. Metabolism is the set of chemical reactions that lets a cell obtain energy and build its own parts. A virus has no ribosomes or complete metabolic system of its own.
Viruses therefore cannot make proteins or obtain the energy needed for replication on their own. After a viral genome enters a suitable host cell, the virus uses the cell's ribosomes and energy. Enzymes from the virus, the host cell, or both copy the viral genome. Host-cell ribosomes make viral proteins, and the new components are assembled into virions.
Outside a host cell, a virion may persist for a while, attach to a suitable cell, and deliver its genome. Replication begins after the virus gains access to the host cell's ribosomes and energy and the enzymes needed to copy its genome.
Viral Activity inside and outside Cells
Outside a cell, a virion does not perform metabolism, make proteins, or copy its genome. Inside a suitable cell, the viral genome can direct the production of viral proteins and new genome copies. The virus therefore reproduces only in a compatible host cell.
Capsid and Envelope Structures Determine Viral Shape
Viruses span a wide size range. Many familiar virions are tens to hundreds of nanometers across, although giant viruses can be larger. A nanometer is one thousandth of a micrometer. Because many virions are below the resolving power of an ordinary light microscope, their detailed structure is studied with methods such as electron microscopy.
Viral shapes differ because capsomeres and outer layers are arranged in different ways. Some viruses are helical, some are polyhedral, some are enveloped and rounded, and some have complex forms such as bacteriophages that infect bacteria. A virus's shape affects how it protects its genome, attaches to a cell, and enters it.
Helical capsids arrange repeated proteins around the genome, polyhedral capsids form facets, enveloped viruses add a lipid layer that may carry projections, and complex viruses can have heads and tails.
- What to examine
- Notice whether the genome is held by repeated capsid units, a faceted capsid, a lipid envelope, or a special attachment structure.
- Biological meaning
- Capsid and envelope structures protect the genome, while surface proteins bind compatible molecules on target cells.
Before many viruses can enter a host cell, a viral surface structure must bind a compatible molecule on that cell. Depending on the virus, this cell-surface molecule can be a protein, a sugar chain, or a sugar attached to a lipid. Some viruses need more than one molecular contact before entry.
Surface Proteins and Host Cell Entry
The genome carries genetic information for viral components. The capsid protects that genome. The binding specificity of surface proteins partly determines which cells the virus can enter.
Viral reproduction follows this order. The genome first enters a suitable cell. Enzymes from the virus, the host cell, or both copy the genome. Host-cell ribosomes make viral proteins, and those proteins are packaged with the genome into new virions.