Genome architecture as the basis of taxonomy
The fundamental criterion for virus taxonomy is a detailed characterization of their genetic apparatus—the nucleic acid (NA). First, the type of molecule is assessed: the genome may be represented by either DNA or RNA. Next, the structural diversity of the nucleic acids is evaluated according to several key parameters. The number of strands is of paramount importance, distinguishing single-stranded and double-stranded viral genomes.
For single-stranded molecules, polarity is a critically important characteristic. There are agents with positive polarity (+), negative polarity (–), as well as mixed-polarity variants known as ambisense viruses (+/–). Topology (the spatial shape) of the molecule plays an important role: it can be elongated linear or closed circular (ring). In addition, the integrity of the genetic material is assessed, which can be either intact (non-segmented) or divided into separate segments.
Additional taxonomic markers
In addition to nucleic acid properties, a wide range of additional characteristics is used for classification. Morphological traits include the overall virion size, the type of symmetry of its structure, and the exact number of capsomeres forming the capsid. Special attention is paid to the presence or absence of an outer lipid envelope—the supercapsid (envelope).
Researchers also rely on the physicochemical properties of viral particles. For example, an important differential feature is the sensitivity of the virion to deoxycholate and ether. Equally significant are the biological properties of the infectious agent: the specifics of its antigenic structure and the preferred site of viral replication inside the infected cell.
Taxonomy, nomenclature, and the Baltimore system
In modern scientific taxonomy, viruses are conventionally grouped based on the mechanisms of messenger (matrix) RNA synthesis. This concept is closely related to the classification proposed by Nobel laureate David Baltimore. The key principle of the Baltimore system is the division of viruses into groups depending on the pathways of mRNA synthesis, which largely determines the logic of intracellular parasitism.
An interesting feature of viral taxonomy is the complete absence of binary nomenclature, which is universally applied to bacteria (double Latin names are not used). Instead, a strict system of suffixes for taxonomic ranks is used:
- Membership in a family is indicated by the ending -viridae.
- Subfamilies are assigned the suffix -virinae.
- Genus names end in -virus.
Classification of major pathogens and their clinical role
Viruses infect all life forms, including plants and bacteria (bacteriophages). For humans, they are the main causative agents of infectious pathologies and also actively participate in carcinogenesis. Many of them are capable of causing post-infectious complications: myocarditis, pancreatitis, and various immunodeficiency states.
A number of viruses are capable of vertical (transplacental) transmission, causing fetal damage. Clear examples are cytomegalovirus and the rubella virus (belonging to the family Togaviridae).
Of immense clinical significance is the group of negative-sense single-stranded RNA viruses, which includes pathogens of the most dangerous infections:
- Bornaviridae: the agent of Borna disease.
- Filoviridae: Ebola and Marburg viruses (causative agents of African hemorrhagic fevers).
- Paramyxoviridae: mumps virus, parainfluenza viruses, respiratory syncytial virus, and measles virus (can lead to subacute sclerosing panencephalitis).
- Rhabdoviridae: rabies virus.
- Orthomyxoviridae: influenza viruses A, B, and C.
- Bunyaviridae: causative agents of Crimean-Congo hemorrhagic fever and hemorrhagic fever with renal syndrome (HFRS).
- Arenaviridae: lymphocytic choriomeningitis virus.
- Deltavirus: defective hepatitis D virus requiring coinfection with hepatitis B virus.
A special category consists of viruses with a reverse transcription mechanism. These are RNA-containing retroviruses (family Retroviridae), which include HIV (the causative agent of AIDS), as well as DNA viruses of the family Hepadnaviridae, a typical representative of which is the hepatitis B virus.
Briefly: answers to topical questions
What classes of viruses are distinguished according to the Baltimore classification?
According to the Baltimore classification, viruses are divided into seven groups depending on the type of genome and the mode of replication.
- Group I — double-stranded DNA.
- Group II — single-stranded DNA.
- Group III — double-stranded RNA.
- Group IV — single-stranded positive-sense RNA.
- Group V — single-stranded negative-sense RNA or dual polarity.
- Group VI — single-stranded positive-sense RNA with reverse transcription (retroviruses).
- Group VII — double-stranded DNA with reverse transcription.
What types of capsid symmetry are found in viruses?
Viruses exhibit three main types of capsid symmetry, which determine the shape of the virion.
- Helical type — a screw-like nucleocapsid structure (e.g., influenza viruses, coronaviruses).
- Icosahedral type (cubic) — an isometrically hollow body containing the nucleic acid inside (e.g., herpes virus).
- Complex type — a third variant of capsid packing structure.
Which viruses are capable of vertical (transplacental) transmission?
Rubella virus and cytomegalovirus are capable of vertical (transplacental) transmission, which can lead to fetal damage.
How do simple viruses differ from complex viruses?
Simple viruses consist of a nucleic acid and a capsid and are non-enveloped. Complex viruses consist of a nucleic acid, a capsid, and a lipoprotein envelope (supercapsid). Simple viruses are characterized by an icosahedral type of symmetry; complex viruses can have icosahedral or helical types. Simple viruses exit the cell by lysis, while complex viruses exit by budding, forming a supercapsid from the host cell membrane. Complex viruses are sensitive to ether.
To memorize suffixes: Family is the longest word (-viridae), subfamily is slightly shorter (-virinae), and genus is simply virus (-virus).
Class and exam questions
Is binary nomenclature applied to viruses?
No, unlike bacteria, viruses do not have a double Latin name. Their taxonomy uses a system of standardized suffixes for genera and families.
What is the basis of the Baltimore classification?
The key principle of this system is the grouping of viruses based on the mechanism of messenger (matrix) RNA synthesis.
By what criteria is the viral genome evaluated?
They take into account the type of nucleic acid (DNA or RNA), the number of strands (one or two), polarity, topology (linear or circular), and the presence of segmentation.
What else to study on this topic
- Mechanisms of reverse transcription in retroviruses and hepadnaviruses
- Pathogenesis of African hemorrhagic fevers (Marburg and Ebola)
- Features of the development of hepatitis B and D coinfection
- Transplacental transmission of viruses and mechanisms of fetal damage
- Mechanisms of viral oncogenesis
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