Viruses are microscopic, non-cellular parasites. They lack the capacity to thrive and reproduce outside of a host body.
|Learning Objectives: To understand viruses and to explain life cycle of a virus.|
In 1898, Friedrich Loeffler and Paul Frosch found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle, virus, smaller than any bacteria.
Most viruses (Figure 1) have either RNA or DNA as their genetic material. They may be single or double-stranded. The entire infectious virus particle, called a virion, consists of nucleic acid and an outer shell of protein called capsid.
A virus that infects bacteria is known as bacteriophage or phage. Although they may seem like living organisms because of their sophisticated reproductive abilities, viruses are considered non-living organisms.
This is because without a host cell, viruses cannot carry out their life-sustaining functions or reproduce.
Viroids are disease-causing organisms that contain only nucleic acid and have no structural proteins.
Prions are virus like particles composed primarily of a protein tightly integrated with a small nucleic acid molecule. They are very resistant to inactivation and appear to cause degenerative brain disease.
Viruses cannot synthesize proteins, because they lack ribosomes. They also derive energy from the host cell.
This is what you will learn:
Structure of a Virus
The infective form of virus, virion, exists outside the host organism. They consist of a genetic material, and an outer protective layer called capsid, collectively known as nucleocapsid.
Small viruses, e.g., polio and tobacco mosaic virus, can also be crystallized. Some virus families have an additional covering, called envelope, that is derived from the modified host cell membranes.
Genetic Material (DNA/RNA)
Most viruses carry single-stranded RNA as their genetic material. RNA viruses comprise of 70% of all viruses.
DsRNA viruses, e.g., members of the reovirus family, contain separate genome segments coding for enzymes involved in RNA replication, major capsid proteins, and several smaller structural proteins.
Most DNA viruses contain a single genome of linear dsDNA. Papovaviruses, such as, polyomaviruses and papillomaviruses, however, have circular DNA genomes (Figure 2).
Viruses containing different genetic materials are shown below:
- dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)
- ssDNA viruses (e.g. Parvoviruses)
- dsRNA viruses (e.g. Reoviruses)
- ssRNA viruses (e.g. Picornaviruses, Togaviruses, Orthomyxoviruses, Rhabdoviruses)
- ssRNA (e.g. Retroviruses)
Capsid is a protein layer made up of subunits called capsomeres, which are formed by smaller subunits called protomers. Capsid proteins are coded for by the virus genome.
Multiple protein copies self-assemble to form continuous three-dimensional capsid structure. Main functions of capsid are as follows:
- Capsid protects viral genome from nucleases
- It attaches to specific receptors exposed on the prospective host cell
- It provides proteins that enables virion to penetrate through the surface of cell membrane
It resembles the lipoprotein bilayer, which is formed by host cell-derived lipid layer. This lipid layer closely surrounds the shell of virus-encoded membrane-associated proteins.
Enveloped viruses often exhibit a fringe of glycoprotein spikes or knobs, also called peplomers. These peplomers aid in the attachment of the viruses to the specific host surfaces.
Outer capsid and envelope proteins of viruses are glycosylated and are important in determining the host range and antigenic composition of the virion (Figure 3).
The structure of a virus can be one of the following types: icosahedral, enveloped, complex, and helical (Figure 4).
Mostly the capsids of sphere-like viruses have symmetry of an icosahedron and are composed of coat proteins. An icosahedral is a polygon with 12 vertices (corner), 20 facet (sides) and 30 edges.
It is the most stable form and are found in human pathogenic virus e.g. polio virus (Figure 5), adenovirus, papovavirus, herpes virus (Figure 6), etc.
Some species of virus envelop themselves in a modified form of cell membranes. They differ from non-enveloped/naked forms.
They are composed of several separate capsomere with varied shapes and symmetries. Poxvirus belongs to this group.
This is an icosahedron form elongated along one axis. E.g. Bacteriophages
Helical capsid are shaped like hollow tubes with protein walls. These viruses are composed of a single type of capsomere.
Capsomeres are stacked around a central axis to form a helical structure with nucleic acid. E.g. Tobacco mosaic virus (TMV), Orthomyxoviridae (causes influenza).
Table showing difference between non-enveloped and enveloped viruses
|Capsid||Outermost covering is capsid and is made of proteins||Outermost envelop is made of phospholipids, proteins, and glycolipids which surrounds the capsid|
|Virulence||Virulent and causes host cell lysis. Infectious even after drying||Less virulent and are released by budding; rarely causing host cell lysis. Loses infectivity after drying|
|Sensitivity||Resistant to heat, acids, and drying. The virus is more powerful and proliferates rapidly in an acidic environment||Sensitive to heat, acids, and drying|
|Survival||Survive inside gastrointestinal tract. Hence, usually, intestinal infections are caused by these viruses||Cannot survive in gastrointestinal tract. Bile salts in the GI tract tend to show detergent-like activity, which can destroy these viruses|
|Sterilization||Difficult to sterilize, as they can easily adjust to changes in temperature||Do not show much resistance to desiccation and heat treatment, hence they are easier to sterilize|
|Immune system||Induces antibody production||Induces cell mediated immune response|
|Infections||Usually do not cause recurrent infection||Potent at attacking the immune system|
|Transmission||Through feces or mouth, and dust||Through blood and organ transplant|
|Examples||Norovirus, Rotavirus, Poliovirus (Figure 5)||Chickenpox virus, Herpes simplex virus (Figure 6), Ebola virus|
Classification of Viruses
Viruses are classified based on their morphology, chemical composition, and mode of replication. Their morphology provides the basis for grouping viruses into families.
They are named according to their presence and structure. For instance, the name Reoviridae is derived from respiratory, enteric, and orphan viruses as they were found in both respiratory and enteric specimens.
Similarly, Retrovirus is from reverse transcriptase and Filoviridae is derived from the Latin filum meaning “thread” or “filament”.
Several viruses are still unclassified due to difficulty in obtaining sufficient samples.
Things to Remember
- Viruses are microscopic, non-cellular parasites.
- The name ‘virus’ came from a Latin word virus which means venom or poisonous fluid.
- The viruses consist of two parts: the centrally placed nucleic acid, covered by protein coat.
- They are transmitted very easily from one organism to another organism.
- Viruses are responsible for various diseases of both plants (tobacco mosaic, leaf roll of potato, leaf curl of papaya etc.) and animals (small pox, meningitis, pneumonia, mumps, bronchitis etc.).