Viruses are genetic elements that cannot replicate independently of a living host cell. Host cells provide energy, metabolic intermediates needed for replication and also synthesize viral proteins. As viruses need suitable living cells to multiply, viruses are called obligate intracellular parasites.
Viruses exist in either extracellular or intracellular forms. The extracellular forms of a virus is called a virus particle or virion, which is a microscopic particle-containing nucleic acid surrounded by a protein coat. Virion is metabolically inert and cannot generate energy or carry out biosynthesis. Virus particle facilitates transmission of virus from one host cell to another.
The entire virus, including nucleic acid, capsid, envelope, and glycoprotein spikes, is called the virion, or a virus particle.
Once a virus enters a new host cell, its intracellular state begins and virus starts replication. Using host cells’ structural and metabolic components, virus forms new copies of virus genomes, components of virus coat, and does assembly of new virions. Eventually, progeny viruses leave the host cell either by budding or the lysis of the host cell.
Size and Shape of Viruses
Virions come in many sizes and shapes. Most viruses are smaller than prokaryotic cells, ranging in size from 0.02 to 0.3 μm (20–300 nm) in diameter. Because of this small size, viruses can pass through bacterial filters and can not be visualized by a light microscope.
Viruses are extremely small, so the common unit of measure for viruses is the nanometer, which is one-thousandth of a micrometer.
- Smallpox virus, one of the largest viruses, is about 200 nm in diameter (about the size of the smallest cells of Bacteria).
- Poliovirus, one of the smallest viruses, is only 28 nm in diameter (about the size of a ribosome).
Most of the animal viruses are roughly spherical with some exceptions.
- Rabies virus: Bullet shaped
- Ebola virus: Filamentous shaped
- Poxvirus: Brick shaped
- Adenovirus: Space vehicle shaped
The structures of virions are quite diverse, varying widely in size, shape, and chemical composition.
Viruses are composed of a nucleic acid genome surrounded by a protein shell called a capsid. Together the genome and capsid are referred to as the nucleocapsid.
This protein coat is composed of a number of individual protein molecules called capsomers. A few viruses have only a single kind of protein in their capsid, but most viruses have several distinct proteins. Capsomers are arranged in a precise and highly repetitive pattern around the nucleic acid. The capsomere is the smallest morphological unit that can be seen with the electron microscope.
A single virion can have a large number of capsomeres. The information for proper folding and assembly of the proteins into capsomeres is typically contained within the structure of the proteins themselves; hence, the overall process of virion assembly is called self-assembly. The complete complex of nucleic acid and protein packaged in the virion is called the virus nucleocapsid.
The nucleocapsids of viruses are constructed in highly symmetric ways. Three kinds of symmetry are recognized in viruses, helical, icosahedron, and complex.
- Helical symmetry: Rod-shaped viruses have helical symmetry. For example, tobacco mosaic virus (TMV), measles, mumps, influenza, rabies, etc.
- Icosahedral symmetry: The icosahedron pattern is the most efficient arrangement for subunits in a closed shell. Spherical viruses have icosahedral symmetry. It is a symmetric structure roughly spherical in shape and contains 20 faces (each an equilateral triangle). There are exactly 60 identical subunits on the surface of an icosahedron. Most viruses are built with icosahedral symmetry. For example, polioviruses, adenoviruses, etc.
- Complex symmetry: These are viruses with complex or uncertain symmetries. For example, smallpox virus has the most complex virion structure consisting of many different proteins and lipoproteins. Bacteriophages are the most complicated viruses in terms of structure as they contain icosahedral heads and helical tails.
Although viruses are acellular entity, they possess a genome that encodes information required for viral replication. Viral genomes are smaller than those of most cells. The largest known viral genome is that of Mimivirus which consists of 1.18 Mbp of double-stranded DNA and is larger than some cellular genomes. Some viruses have genomes so small they contain fewer than five genes.
Viruses have either DNA or RNA genomes (in contrast, all cells contain double-stranded DNA genomes). Virus can be classified according to whether the nucleic acid in the virion is DNA or RNA and further subdivided according to whether the nucleic acid is single (ss) or double-stranded (ds), linear, or circular.
Most of the viral genomes are linear but some viral genomes are circular. Viruses whose genomes consist of DNA follow the central dogma of molecular biology but RNA viruses are exceptions to this rule. Regardless of the genome structure, all viruses must synthesize messenger RNA (mRNA) which is then translated by the host cells’ translational machinery (ribosomes).
Some viruses are naked (non-enveloped viruses are called naked), whereas others possess lipid-containing layers around the nucleocapsid called an envelope.
Enveloped viruses contain a membrane surrounding the nucleocapsid. The viral envelope consists of a lipid bilayer, derived from the membranes of the host cell. Embedded in it are viral membrane proteins, usually, glycoproteins, coded by viral genes. Glycoprotein spikes extend from the surface of the virus and act as attachment projections or as enzymes (e.g., neuraminidases).
As the envelope of a virion makes initial contact with the host cells, it controls the specificity of virus infection. The virus-specific envelope proteins are critical for attachment of the virion to the host cell during infection. Virus attaches to specific receptors on the host cell membrane via their glycoprotein spikes. The specificity of this interaction determines the host and cells within the host.
Virions that have envelopes are sensitive to lipid solvents such as ether and chloroform. Their capacity to infect cells is inactivated by these solvents. Naked viruses are not affected by lipid solvents.
Enzymes in Virions
Although most virions lack their own enzymes, some virions contain one or more virus-specific enzymes. Such enzymes play a role during the infection and replication processes. Some of the common viral enzymes and their roles are summarized in the table below.
|Name of the enzyme||Virions carrying it||Functions|
|Lysozyme||Bacteriophage||Bacteriophage use lysozyme to make a small hole in the bacterial cell wall, through which they inject their nucleic acid in the host cell cytoplasm. Lysozyme produced during later stages of bacteriophage infection, helps to lyse the bacterial cell wall and release of the progeny viruses.|
|RNA-dependent DNA polymerase (reverse transcriptase)||Retroviruses||Reverse-transcriptase transcribes the viral RNA to form a DNA intermediate|
|RNA polymerases||RNA viruses||Because cells cannot make DNA or RNA from an RNA template|
|Neuraminidases||Certain animal viruses||Cleave glycosidic bonds in glycoproteins and glycolipids of animal cell connective tissue and aid the release of virions from the host cells.|
References and further readings
- Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e Levinson W, Chin-Hong P, Joyce EA, Nussbaum J and Schwartz B. Lange, Mc Graw Hill.
- Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
- Louten, J. (2016). Virus Structure and Classification. Essential Human Virology, 19–29. https://doi.org/10.1016/B978-0-12-800947-5.00002-8