Viruses-Structure and Nature of Viruses

The Nature of Viruses

All viruses have the same basic structure consisting of a core of nucleic acid surrounded by protein. This structure lacks cytoplasm, and it is not a cell. Individual viruses contain only a single type of nucleic acid, either DNA or RNA. The DNA or RNA genome may be linear or circular; single-stranded or double-stranded.
RNA viruses may be segmented, with multiple RNA molecules within a virion, or nonsegmented, with a single RNA  molecule. Viruses are classified, in part, by the nature of their genomes:

RNA viruses, DNA viruses, or retroviruses.

The structure of a typical myovirus bacteriophage

Protein coat

Nearly all viruses form a protein sheath, or capsid, around their nucleic acid core (figure 27.1). The capsid is composed of one to a few different protein molecules repeated many times.  The repeating units are called capsomeres. In several viruses, specialized enzymes are stored with the nucleic acid, inside the capsid.

One example is reverse transcriptase, which is required for retroviruses to complete their cycle and is not found in the host. This enzyme is needed early in the infection process and is carried within each virion.

Many animal viruses have an envelope around the capsid that is rich in proteins, lipids, and glycoprotein molecules. The lipids found in the envelope are derived from the host cell; however, the proteins found in a viral envelope are generally virally encoded.

Viral hosts

Viruses occur as obligate intracellular parasites in every kind of organism that has been investigated for their presence. Viruses infect fungal cells, bacterial cells, and protists as well as cells of plants and animals, however, each type of virus can replicate in only a very limited number of cell types.

A virus that infects bacteria would be ill-equipped to infect a human or plant cell. The suitable cells for a particular virus are collectively referred to as its host range.

Once inside a multicellular host,  many viruses also exhibit tissue tropism, targeting only a specific set of cells. For example, the rabies virus grows within neurons, and the hepatitis virus replicates within liver cells.

Once inside a host cell, some viruses, such as the highly dangerous Ebola virus, wreak havoc on the cells they infect; others produce little or no damage. Still, other viruses remain dormant until a specific signal or event triggers their expression.

As one example, a person can get chicken pox as a child, recover, and develop the disease shingles decades later. Both chicken pox and shingles are caused by the same virus, varicella zoster. This virus can remain dormant, or latent, for years.

Stress to the immune system may trigger an outbreak of shingles in people who have had chicken pox in the past. This is caused by the same virus, but the infection may be called herpes zoster because the virus is actually a herpes virus.

Shapes of Viruses:

Most viruses have an overall structure that is either helical or icosahedral. Helical viruses, such as the tobacco mosaic virus in figure a, have a rodlike or threadlike appearance. Icosahedral viruses have a soccer ball shape, the geometry of which is revealed only under the highest magnification with an electron microscope.

The icosahedron is a structure with 20 equilateral triangular facets. Most animal viruses are icosahedral in basic structure (figure b). The icosahedron is the basic design of the geodesic dome, and it is the most efficient symmetrical arrangement that subunits can take to form a shell with maximum internal capacity (figure ). Some viruses, such as the T-even bacteriophage shown in the figure, are complex.

Complex viruses have a binal, or two-fold, symmetry that is not either purely icosahedral or helical. The T-even phage shown has a head structure that is an elongated icosahedron. A collar connects the head to a hollow tube with helical symmetry that ends in a complex baseplate with tail fibers. Although animal viruses do not have this binal symmetry, some, such as the poxviruses, do have a complex multilayered capsid structure.

Some enveloped viruses, such as influenza, are polymorphic, having no distinctive symmetry. Viruses also vary greatly in size. As shown in the figure, the very smallest viruses, such as the poliovirus, have actually been synthesized in a lab using nothing more than sequence data and a machine capable of synthesizing nucleic acids from nucleotides.

The larger viruses, such as the poxviruses, generally carry more genes, have more complex structures, and tend to have a very short cycle time between the entry of viral particles and the release of newly formed virions.

Important Human Viral Diseases

There are many viral diseases that can affect humans. Some of the most important ones include:

  1. Influenza (the flu) – a respiratory illness caused by the influenza virus. It can cause symptoms such as fever, cough, body aches, and fatigue, and in severe cases can lead to hospitalization or death.
  2. Coronavirus disease (COVID-19) – a respiratory illness caused by the SARS-CoV-2 virus. It first emerged in late 2019 and has since become a global pandemic, leading to widespread illness and death.
  3. HIV/AIDS – a viral infection that attacks the immune system and can lead to acquired immune deficiency syndrome (AIDS), a condition in which the body is unable to fight off infections and diseases. HIV is transmitted through bodily fluids such as blood, semen, vaginal secretions, and breast milk.
  4. Hepatitis B – is a viral infection that affects the liver and can cause inflammation, scarring, and in severe cases, liver failure. It is transmitted through contact with the blood or bodily fluids of an infected person.
  5. Herpes simplex virus (HSV) – a viral infection that can cause sores and blisters on the skin and mucous membranes. It is most commonly transmitted through sexual contact, but can also be transmitted through oral-to-oral contact or through contact with herpes sores.
  6. Ebola virus disease – a viral illness that causes severe illness and often death. It is transmitted through contact with the blood or bodily fluids of an infected person or animal.
  7. Measles – is a highly contagious viral illness that can cause fever, rash, and respiratory symptoms. It is transmitted through the air when an infected person coughs or sneezes.



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