Specificity of the viral genome
The main and absolutely unique feature of viruses in the context of genetics is the nature of their heredity carrier. In all cellular life forms known to science, genetic information is strictly encoded in DNA. Viruses, however, demonstrate exceptional diversity: their genome can be represented by either a DNA molecule or an RNA molecule. This feature is the cornerstone of viral heredity and determines many mechanisms of their variability and survival.
Mutational process in viruses
Like any other biological objects, viruses are subject to mutations. This process ensures their continuous evolution.
Based on the causes of occurrence, the mutational process is divided into two categories:
- Spontaneous mutations: occur naturally. Most often, these are mundane errors that happen during the replication (doubling) of the viral genome.
- Induced mutations: occur under the targeted or accidental exposure to external factors—mutagens. Interestingly, viruses are affected by the same mutagens as bacterial cells.
Phenotypic manifestations of mutations Changes in the genetic code inevitably lead to changes in the external or functional traits of the virus (phenotype). In virology, several key manifestations of such shifts are distinguished:
- Change in antigenic structure: the virus begins to synthesize different proteins, making it "invisible" to the immune system previously familiar with the original strain.
- Loss of capacity for productive infection: the mutant virus loses the ability to replicate inside the host cell.
- Temperature sensitivity: so-called ts-mutations (temperature-sensitive). The virus loses the ability to function or replicate normally at certain, usually elevated, temperatures.
- Change in plaque morphology: when growing viruses in a cell culture, they form so-called "plaques" (zones of destroyed cells). Due to mutations, these plaques can change their shape or size, which is easily detected by a researcher under a microscope.
Mixed infection (Co-infection)
Virus variability can occur not only through point mutations, but also as a result of large-scale interactions. This happens during a mixed infection, when a single susceptible cell is simultaneously infected by several completely different viruses.
A critically important condition for the development of such processes is the meeting of different viral particles at the level of a single cell (and not just a single organism or a single virion). In such a close environment, special mechanisms of trait exchange are triggered.
Phenotypic mixing
One of the most striking results of co-infection is phenotypic mixing.
Conditions of occurrence: the phenomenon is observed exclusively during the mixed infection of a single susceptible cell by two different viral agents.
Essence of the phenomenon: during the assembly of new viral particles inside the cell, confusion occurs. As a result, part of the forming viral progeny acquires a mixed phenotype—that is, it inherits the physical traits of both parental viruses simultaneously.
Genetic status: the most important feature of this process is that the true genotype of the virus does not change. The hereditary information remains strictly original. Changes are exclusively superficial and concern only the structure of the envelope or capsid proteins. That is, a virus with genome "A" can accidentally "put on" a protein coat partially consisting of proteins from virus "B", but inside it will remain virus "A".
Briefly: answers to topical questions
What specific factors act as mutagens causing induced mutations in viruses?
Induced mutations in viruses are caused by exposure to mutagens similar to bacterial ones. Depending on the nature of the exposure, three groups of mutagens are distinguished: physical, chemical, and biological. Physical mutagens include γ-radiation and ultraviolet radiation. Chemical mutagens include base analogs (purine and pyrimidine), as well as nitrous acid and its derivatives. Biological mutagens are transposons—mobile genetic elements.
How does phenotypic mixing differ from genetic recombination in viruses?
Phenotypic mixing occurs during the mixed infection of a single susceptible cell by two different viruses. Part of the viral progeny acquires the phenotypic traits of both parental viruses, but the genotype does not change; changes affect the envelope or capsid proteins. Genetic recombination consists in the exchange between homologous regions of genomes.
What is genetic complementation during a mixed viral infection?
Genetic complementation is the interaction of viruses at the level of protein products, in which the genome of the viruses themselves does not change. In this process, one mutant virus synthesizes a non-functional protein, and the second helper virus synthesizes a normal protein. As a result, the normal protein of the second virus compensates for the defect of the first virus, thereby ensuring its reproduction, although the genome of the mutant virus remains the same.
What is the essence of the process of genetic reassortment in viruses?
Genetic reassortment consists in the exchange of entire RNA genome segments between different variants of viruses that have simultaneously infected the same cell. This mechanism is typical for viruses with a fragmented or segmented genome, such as the influenza virus or rotaviruses. Reassortment provides viral diversity, the formation of new strains, and serves as one of the main mechanisms of variability.
What is multiple reactivation of viruses?
Genetic reactivation occurs when related viruses that have mutations or defects in different genes meet. The mechanism consists in the redistribution of genetic material between them, as a result of which a full-fledged progeny genome is formed from two defective viruses.
To remember the essence of phenotypic mixing, imagine two people who accidentally swapped jackets in a cloakroom. Their appearance (phenotype) changed due to someone else's clothing (capsid proteins), but their IDs and identity (genotype) remained the same.
Class and exam questions
What is the main difference between the viral genome and the genome of cellular organisms?
The genome of cellular organisms is always represented by DNA, while the carrier of hereditary information in viruses can be either DNA or RNA.
How do ts-mutations manifest in viruses?
These are temperature-sensitive mutations. When they occur, the virus changes its properties (for example, loses the ability to replicate) when the temperature regime changes.
Does the genotype change during phenotypic mixing?
No, the genotype remains strictly original. The mixed phenotype arises only due to changes in the capsid proteins or the outer envelope of the virus.
What is necessary for phenotypic mixing to occur?
A mandatory condition is a mixed infection (co-infection) of a single susceptible cell by two different viruses simultaneously.
What else to study on this topic
- Mechanisms of spontaneous mutagenesis and errors of viral polymerases
- Nature of chemical and physical mutagens acting on viruses
- Methods of virus cultivation and analysis of plaque morphology
- The second mechanism of variability during co-infection (aside from phenotypic mixing)
- Practical significance of ts-mutations in the creation of live vaccines
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