Specifics of the Viral Genome
The primary and most unique characteristic of viruses in the context of genetics is the nature of their heredity carrier. In all known cellular life forms, genetic information is strictly encoded in DNA. Viruses, however, demonstrate exceptional diversity: their genome can be represented by either a DNA or an RNA molecule. This feature is the cornerstone of viral heredity and determines many of their variation and survival mechanisms.
Mutational Process in Viruses
Like any other biological entities, viruses are subject to mutations, which ensure their continuous evolution.
Based on their origin, mutations are divided into two categories:
- Spontaneous mutations: Arise naturally, most commonly as routine errors during the replication (duplication) of the viral genome.
- Induced mutations: Occur due to targeted or accidental exposure to external factors—mutagens. Interestingly, viruses are affected by mutagens similar to those that affect bacterial cells.
Phenotypic Manifestations of Mutations Alteration of the genetic code inevitably leads to changes in the external or functional traits of the virus (phenotype). Virology distinguishes several key manifestations of these shifts:
- Alteration of antigenic structure: The virus begins to synthesize different proteins, making it "invisible" to the immune system previously familiar with the original strain.
- Loss of productive infection capacity: The mutant virus loses the ability to replicate inside the host cell.
- Temperature sensitivity: So-called ts (temperature-sensitive) mutations. The virus loses its ability to function or replicate normally at certain, usually elevated, temperatures.
- Changes in plaque morphology: When viruses are cultured in cell monolayers, they form plaques (zones of lysed cells). Due to mutations, these plaques can alter their shape or size, which is easily visualized under a microscope.
Mixed Infection (Co-infection)
Viral variability can occur not only through point mutations but also as a result of large-scale interactions. This happens during mixed infection, when a single susceptible cell is simultaneously infected by multiple distinct viruses.
A critically important condition for such processes is the meeting of different viral particles at the level of a single cell (rather than just the same organism or virion). In such a confined environment, specialized mechanisms of trait exchange are triggered.
Phenotypic Mixing
One of the most striking results of co-infection is phenotypic mixing.
Conditions for occurrence: This 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 progeny inside the cell, packaging errors occur. As a result, a portion of the forming viral progeny acquires a mixed phenotype—inheriting physical traits from both parental viruses simultaneously.
Genetic status: The crucial feature of this process is that the true viral genotype does not change. The hereditary information remains strictly original. The changes are purely superficial, affecting only the structure of the envelope or capsid proteins. Thus, a virus with genotype "A" may accidentally "wear" a protein coat partially composed of proteins from virus "B", but internally it remains virus "A".