Basic classification criteria
Modern taxonomy of non-cellular life forms is based on a strict set of morphological and biochemical characteristics. Assignment to taxonomic groups is carried out based on three main criteria:
- Type and form of nucleic acid. It determines which specific genetic material the viral particle carries (DNA or RNA), and also evaluates the number of strands in the molecule.
- Capsid symmetry type. The spatial organization of the protein shell protecting the genome is taken into account.
- Presence of an outer envelope. The presence or absence of an additional surface layer—the lipoprotein envelope (supercapsid)—is evaluated.
Architecture and symmetry types
Protein subunits surrounding the genetic material can organize in space in various ways, forming the capsid. The complex consisting of the nucleic acid and protein subunits forms a single structure called the nucleocapsid.
There are two main types of spatial symmetry:
- Helical symmetry. Most characteristic of the vast majority of plant-infecting viruses, and is also found in some bacteriophages. A classic example is the tobacco mosaic virus (TMV). Inside its structure lies helical RNA, and the capsid itself is formed by exactly 2,130 identical protein subunits.
- Cubic symmetry. This shape is typical for the vast majority of viral agents that cause infectious diseases in humans and various animals. The shape of such a capsid is an icosahedron—a regular twenty-sided polyhedron. Geometrically, this figure has 12 vertices, 30 edges, and 20 regular faces. A characteristic representative of pathogens with cubic symmetry is the poliovirus.
Taxonomy by genetic material
Division by the type of nucleic acid and the number of strands is fundamental in virology. All viral agents are divided into two huge groups: DNA-containing and RNA-containing.
DNA-containing viruses are subdivided into:
- Single-stranded: this group includes some bacteriophages.
- Double-stranded: the group includes adenoviruses, the smallpox pathogen, and herpesviruses.
RNA-containing viruses are classified similarly:
- Single-stranded: these include enteroviruses, influenza virus, rabies virus, and the aforementioned tobacco mosaic virus.
- Double-stranded: this category includes oncogenic viruses and HIV (human immunodeficiency virus).
Fundamental differences from cellular life forms
Viruses occupy a special position in biology because they lack a cellular structure. Their fundamental differences from full-fledged cells come down to three key aspects:
- Obligate parasitism. Viral particles do not possess their own metabolism and are never capable of reproducing outside a suitable host cell.
- Unique genetic material. Unlike cells, which always contain both nucleic acids for the realization of genetic information, viruses contain exclusively one type—either only DNA or only RNA.
- Specific mode of reproduction. Viruses never reproduce by binary fission (a process characteristic of bacterial cells).
Briefly: answers to topical questions
Into how many classes are viruses divided according to the Baltimore system?
According to the Baltimore classification, viruses are divided into seven classes. This classification is based on the type of nucleic acid and the replication method (mRNA synthesis mechanism).
The following classes are distinguished:
- I — double-stranded DNA (without an RNA stage)
- II — single-stranded DNA
- III — double-stranded RNA
- IV — single-stranded (+RNA)
- V — single-stranded (–RNA) or double polarity
- VI — single-stranded (+RNA) with a DNA stage
- VII — double-stranded DNA with an RNA stage
What is a supercapsid and what is it composed of?
A supercapsid (or peplos) is an additional outer lipoprotein envelope of complex viruses. It represents a double lipoprotein membrane and is formed by the budding of the virion through the host cell membrane: plasma (cytoplasmic), nuclear, or endoplasmic reticulum membranes.
On the surface of the supercapsid are peplomers—glycoprotein "spikes" for interaction with the cell. In some viruses, a matrix protein (M-protein) is located between the capsid and the supercapsid, linking the supercapsid to the nucleocapsid.
What is the mechanism of viral reproduction inside the host cell?
The process of viral reproduction proceeds in several sequential stages:
- Adsorption — attachment of the virion to the surface of the target cell through the interaction of viral attachment proteins and specific cell receptors.
- Penetration and "uncoating" — entry of the virus into the cell (via endocytosis, membrane fusion, or translocation) and deproteinization (release of the genome from coats).
- Synthesis — replication of viral components (proteins and nucleic acids).
- Assembly — formation of new virions from replicated genomes and capsid proteins.
- Release — departure of new viral particles from the cell (by cell lysis in simple viruses or by budding in complex ones).
To quickly memorize the three main differences between a virus and a cell, use the mnemonic OGP: Obligate parasitism, Genetic material of a single type, Reproduction without division.
Class and exam questions
What is a nucleocapsid?
This is a unified structure of the viral particle formed by a complex of nucleic acid (DNA or RNA) and protective protein subunits.
What geometric shape is characteristic of cubic symmetry?
The capsid with cubic symmetry has the shape of an icosahedron—a regular twenty-sided polyhedron consisting of 12 vertices, 30 edges, and 20 faces.
Can viruses reproduce by binary fission?
No, viruses never reproduce by division. Binary fission is characteristic of cellular organisms, whereas viruses assemble from synthesized components inside the host cell.
What else to study on this topic
- Structural features of the tobacco mosaic virus (TMV) and helical symmetry
- Icosahedral geometry in the structure of the poliovirus
- The role of the outer lipoprotein envelope in classification
- Mechanisms of obligate intracellular parasitism
- Comparative characteristics of single-stranded and double-stranded RNA viruses
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