History of discovery and development of virology
The science that studies non-cellular forms of life is called virology. It owes its emergence to the Russian botanist Dmitry Ivanovsky, who made a breakthrough discovery in 1892.
While studying tobacco mosaic disease affecting tobacco plants, Ivanovsky conducted an elegant experiment. He obtained an infectious extract from the tissues of infected plants and passed it through a special filter. The pores of this filter were so small that they reliably trapped any bacteria known at the time. However, the key fact of the experiment was that the filtered liquid did not lose its properties — it still caused disease in healthy plants. This proved the existence of pathogens significantly smaller in size than bacteria.
Several years later, in 1898, the Dutch researcher Martinus Beijerinck introduced the term "virus" into scientific circulation to describe this mysterious infectious agent. The word itself comes from the Latin virus, which translates as "poison".
Basic properties and mechanism of parasitism
Viruses are microscopic living entities that can only be observed using an electron microscope. Their main characteristic is belonging to non-cellular forms of life and the status of obligate parasites.
Obligate parasitism means that viruses are fundamentally incapable of independent reproduction and vital activity outside a host organism. In the environment, transmitted from one cell to another, they exist as completely inert particles showing no signs of life.
The mechanism of the pathogenic action of a virus unfolds only after successful invasion into a living cell:
- The virus penetrates the host cell.
- The host cell's own DNA is "switched off", and cellular processes come to a halt.
- The virus inserts its own genetic apparatus (DNA or RNA).
- The cell receives a new command and begins continuous synthesis of new viral particles, expending its resources on this, which ultimately leads to the development of the disease.
Forms of existence: virion and intracellular stage
The life cycle of a virus is strictly divided into two fundamentally different phases, depending on its location relative to the host cell.
- Extracellular form (Virion). It is a fully formed, infection-ready particle. At this stage, the virus is outside the living cell and structurally is a nucleoprotein complex. In this form, it travels through space awaiting an encounter with a suitable host.
- Intracellular form. Once inside the host cell, the virus sheds its coat. At this stage, it exists solely in the form of pure nucleic acid, which exerts genetic control over the infected cell.
Hypotheses of virus origin
Since viruses leave no fossil remains, their evolutionary past remains a subject of scientific debate. Today, three main theories of their origin dominate virology:
- Origin from cellular organisms. It is hypothesized that the ancestors of viruses could be ancient bacteria that transitioned to strict intracellular parasitism.
- "Escaped" nucleic acid theory. According to this hypothesis, viruses originated from fragments of nucleic acids that originally belonged to normal cells, but in the course of evolution acquired the ability to replicate (duplicate) completely independently of the cell in which they arose.
- Degeneration theory. This concept states that viruses arose as a result of profound evolutionary degeneration of fully-fledged cellular organisms that lost all organelles except the genetic apparatus as unnecessary.
Distribution in nature
Viruses are characterized by ubiquitous distribution. They are integrated into all biocenoses and are capable of infecting absolutely all known groups of living organisms, from bacteria to mammals.
The diversity of viral agents is colossal. To date alone, about 500 forms of viruses specializing in infecting warm-blooded vertebrates have been described. In addition, more than 300 forms causing various diseases in higher plants are known.
Briefly: answers to topical questions
What components make up the structure of simple and enveloped virions?
Virions are divided into simple and enveloped ones, differing by the presence of an outer lipid envelope.
| Virion type | Components |
|---|---|
| Simple | Genome (DNA or RNA) and capsid (protein coat made of capsomeres). Together they form a nucleocapsid. |
| Enveloped | Nucleocapsid and supercapsid (outer lipoprotein envelope, which is a modified host cell membrane). Spikes (peplomers)—glycoprotein projections—may be present on the surface of supercapsids. In some viruses, a matrix protein (M protein) is located between the capsid and the supercapsid. |
What are the types of viral capsid symmetry?
The capsid forms highly symmetrical structures, which are divided into three main types.
- Helical — the complex of nucleic acid and protein subunits forms a helix (typical of plant viruses).
- Icosahedral — the capsid has the shape of an icosahedron (regular twenty-sided polyhedron), which is typical of most human and animal viruses.
- Complex — a combined type of symmetry found in bacteriophages (an icosahedral head containing the genetic material and a helical tail).
What stages are included in the complete life cycle (reproduction) of a virus in a host cell?
The complete viral reproduction cycle inside the cell includes six consecutive stages.
- Adsorption — attachment of viral particles to specific receptors on the cell surface.
- Penetration and uncoating — in bacteriophages, the nucleic acid is injected into the cell; in other viruses, endocytosis of the entire viral particle occurs, followed by release of the nucleic acid from the protein capsid.
- Replication of viral nucleic acid — occurs using the pool of nucleotides accumulated in the host cell.
- Synthesis of viral proteins and enzymes — carried out on the ribosomes of the host cell.
- Assembly of viral particles — formation of new virions from the synthesized viral nucleic acids and viral proteins.
- Lysis (release of viral particles) — in bacteria, it is accompanied by cell destruction; in eukaryotes, it occurs via budding or extrusion of particles into the extracellular environment.
Into which classes are viruses divided according to the Baltimore classification?
According to the Baltimore classification, which is based on the replication mechanism and the type of nucleic acid, viruses are divided into seven classes.
- Class I — double-stranded DNA.
- Class II — single-stranded DNA.
- Class III — double-stranded RNA.
- Class IV — single-stranded (+RNA).
- Class V — single-stranded (–RNA) or ambisense.
- Class VI — single-stranded (+RNA) with a DNA intermediate stage (retroviruses).
- Class VII — double-stranded DNA with a DNA synthesis stage via an RNA intermediate.
To easily recall the three hypotheses of viral origin, use the acronym DBK: Degeneration of cells, Bolting (escaped) nucleic acid, from Cellular organisms (bacteria).
Class and exam questions
What is a virion?
A virion is the extracellular form of a virus. It is a fully formed, inert infectious particle (nucleoprotein complex) capable of infecting a new cell.
What is the genetic parasitism of viruses?
Upon entering the cell, the virus "shuts down" the host DNA and uses its own nucleic acids (DNA or RNA) to force the cell to synthesize exclusively new viral particles.
What else to study on this topic
- Architecture and chemical composition of the virion nucleoprotein complex
- Classification of viruses by nucleic acid type (DNA and RNA-containing)
- Molecular mechanisms of host cell DNA "shutdown"
- Features of viral diseases in warm-blooded vertebrates
- Specifics of viral penetration into higher plant cells
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Viruses: Non-Cellular Forms of Life" and 3,793 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in the section "The Cell — The Elementary Unit of Life"
Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.