Fundamental Differences and Virion Morphology
A mature, fully formed viral particle is called a virion. As "filterable agents", viruses are comparable in size to the thickness of a bacterial cell wall. The smallest are the causative agents of poliomyelitis and parvoviruses (about 18–20 nm), while the largest are poxviruses (e.g., smallpox virus), reaching up to 350 nm in length.
Virions are extremely diverse in shape. Several main morphological types are distinguished:
- Spherical (characteristic of HIV, polioviruses, herpesviruses).
- Rod-shaped (the classic example is the tobacco mosaic virus).
- Bullet-shaped (rounded at one end and flat at the other, as in the rabies virus).
- Filamentous or thread-like (long convoluted structures characteristic of filoviruses).
- Spermatozoon-like (characteristic structure of many bacteriophages).
Genetic Organization
The viral genome is represented by DNA or RNA, which can be linear, circular, or fragmented. In terms of ploidy, the absolute majority of viruses are haploid (have a single set of genes), and only retroviruses possess a diploid genome. The volume of genetic information ranges from a few units to hundreds of genes.
Inside an infected cell, the viral genome can exist as a provirus (when the genetic material is integrated into the host chromosome) or in a plasmid-like form (autonomous presence of the nucleic acid in the cytoplasm, which is typical for herpesviruses).
The polarity of single-stranded RNA-containing viruses is of particular importance:
- Plus-strand (+RNA): performs both the hereditary function and the role of messenger RNA. Such a strand is infectious on its own and capable of directly initiating protein translation on host ribosomes.
- Minus-strand (-RNA): carries exclusively hereditary information. For protein synthesis to begin, a complementary plus-strand must first be formed on its template.
- Ambisense viruses: the genome has regions of both polarities.
Principles of Structural Classification
Basic taxonomy is based on three main criteria: the type of nucleic acid (DNA or RNA), genome structure (single- or double-stranded), and the presence of an outer lipoprotein envelope.
- Simple ("naked") viruses consist only of nucleic acid and a protective protein coat — the capsid. The capsid itself is formed from individual subunits (capsomeres). Examples include hepatitis A and polioviruses.
- Enveloped viruses have an additional layer over the nucleocapsid — the supercapsid (envelope), which is often equipped with special spikes. Enveloped viruses include coronaviruses, orthomyxoviruses, and herpesviruses.
Key Families of Pathogenic Viruses
Viral taxonomy includes several large clinically significant groups.
DNA virus group: Among non-enveloped double-stranded viruses, Adenoviridae (causing ARVI), Papillomaviridae (warts and oncology), and Polyomaviridae are distinguished. Non-enveloped single-stranded DNA viruses include the smallest Parvoviridae (causative agents of erythema infectiosum) and Anelloviridae. Among enveloped DNA viruses, the most important are the complex and large Poxviridae, spherical Herpesviridae (herpes simplex viruses, cytomegalovirus, Epstein-Barr virus, Varicella Zoster), and small Hepadnaviridae.
RNA virus group: Double-stranded non-enveloped viruses include Reoviridae, in particular, rotaviruses (the main cause of gastroenteritis). Non-enveloped single-stranded viruses are represented by small families Picornaviridae (hepatitis A), Caliciviridae (noroviruses), Hepeviridae i Astroviridae. Enveloped single-stranded RNA viruses represent the largest group. This includes spherical forms: Coronaviridae (severe ARVI pathogens), vector-borne Flaviviridae (yellow fever, tick-borne encephalitis, hepatitis C), Orthomyxoviridae, Retroviridae i Togaviridae. Bullet-shaped ones include Rhabdoviridae, and filamentous ones — Filoviridae.
Briefly: answers to topical questions
What stages does the viral reproduction cycle in the host cell include?
The viral reproduction cycle in the host cell includes five sequential stages.
- Adsorption — attachment of the virion to the surface of the target cell.
- Penetration and "uncoating" — entry of the virus into the cell and deproteinization (release of the genome from its 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.
What types of capsid symmetry are found in viruses?
Viruses exhibit three types of nucleocapsid symmetry:
- Helical — capsomeres form a spiral-like structure. Characteristic of rod-shaped, bullet-shaped, and filamentous forms.
- Cubic — capsomeres are arranged in the form of a polyhedron (dodecahedron or icosahedron); virions take on a spherical shape.
- Mixed (combined) — characteristic of bacteriophages: the head has cubic symmetry, and the tail has helical symmetry.
What is the chemical composition and origin of the supercapsid in enveloped viruses?
The supercapsid (peplos) of enveloped viruses is chemically a double lipoprotein membrane on the surface of which glycoprotein "spikes" (peplomers) are located. The envelope is of cellular origin and is formed by the budding of the virion through the membranes of the host cell:
- Plasma membrane
- Nuclear membrane
- Endoplasmic reticulum membranes
The presence of lipids makes such viruses sensitive to fat solvents (ether).
To remember the difference between a simple virus and a complex one, imagine winter clothing: a "naked" virus wears only a sweater (capsid), while a "complex" one also puts on a jacket (supercapsid) over it.
Class and exam questions
What is disjunctive reproduction?
This is a dissociated reproduction pathway in which viral nucleic acids and proteins are synthesized in different parts of the cell and then assembled into a single viral particle (virion).
How do +RNA viruses differ from -RNA viruses?
Plus-strand RNA can immediately act as a template (mRNA) for protein synthesis on ribosomes, which is why it is initially infectious. Minus-strand RNA must first be synthesized into a plus-strand.
What is the difference between a provirus and a plasmid-like form?
The provirus is tightly integrated into the host cell genome. The plasmid-like form exists autonomously in the cell cytoplasm without integrating into chromosomes.
What else to study on this topic
- Stages of disjunctive viral reproduction in the cell
- Mechanisms of assembly of simple and complex virions
- Features of ambisense viruses
- Morphology and clinical significance of bacteriophages
- The role of retroviruses and features of their diploid genome
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