Virion core and genetic material
The central part of any viral particle is its core, which contains the genetic material. The space surrounding the genetic apparatus inside the protein coat forms the so-called internal environment of the virus.
The fundamental rule of virology is that viruses always contain exclusively one type of nucleic acid. This means that a virion can be either DNA-containing or RNA-containing, but never contains both molecules simultaneously.
The structural organization of viral nucleic acids exhibits great diversity. Genetic material can be represented by either single-stranded or double-stranded molecules. Furthermore, the spatial form of the nucleic acid can be linear or coiled into a circular structure. For example, the genetic apparatus of a virion may be represented by two identical RNA molecules, each containing a specific set of genes (in some cases, nine genes per molecule).
Structure and functions of the capsid
The genetic material of the virion is surrounded by a specialized protein coat called the capsid. This coat has a complex yet strictly ordered structure. It is formed by multiple identical repeating structural subunits called capsomeres.
The spatial organization of capsomeres creates structures with a very high degree of symmetry. Due to this regular architecture, viral particles possess a unique ability to crystallize. There are two main types of spatial symmetry in viral capsids:
- Helical symmetry.
- Cubic symmetry.
The capsid performs tasks that are critically important for the virus's survival. First, it acts as a reliable physical and chemical barrier. The protein shell protects vulnerable viral DNA or RNA from the harmful effects of ultraviolet radiation as well as from cleavage by nuclease enzymes present in the environment. Second, the capsid plays a key role in the initial stages of the infectious process, ensuring the attachment (or adsorption) of the viral particle to the surface of the host cell.
Envelope: a feature of complex viruses
In addition to the basic protein capsid, some highly organized viruses possess an additional outer shell—the viral envelope. The presence of this structure is characteristic of such dangerous pathogens as human immunodeficiency virus (HIV), influenza virus, various herpesviruses, and other complex virions.
The envelope is a lipoprotein membrane. Its origin is a crucial fact: the virus does not synthesize the lipid bilayer on its own, but rather borrows it from the plasma membrane of the infected host cell (for example, during budding from a lymphocyte).
Specific viral structures—glycoproteins—are embedded into this borrowed lipid bilayer membrane. Visually, they often resemble mushroom-like projections or spikes on the virion surface. These receptor structures are critically important for recognizing target cells and subsequently penetrating them.
Enzymatic composition of the virion
Some virions carry beneath their coats not only genetic information, but also a ready-made set of specialized enzymes required to initiate the life cycle within an infected cell. The enzymatic composition of the core may include:
- Reverse transcriptase (revertase). This is a unique enzyme that catalyzes the process of reverse transcription—the synthesis of viral DNA on a viral RNA template. This process is triggered immediately after the contents of the virion enter the cytoplasm of the host cell.
- Protease. An enzyme involved in cleaving long protein precursors into functional viral proteins.
- Integrase. An enzyme that ensures the insertion (integration) of the newly synthesized viral DNA into the chromosomes of the infected cell.
Briefly: answers to topical questions
Which specific cell receptors do human immunodeficiency virus glycoproteins bind to?
Human immunodeficiency virus glycoproteins bind to specific CD4 cell receptors.
- CD4 is the main receptor on the surface of target cells to which the viral glycoprotein gp120 specifically binds.
- Chemokine receptor CCR5 is used by the virus as a coreceptor for recognition and entry on the surface of lymphocytes.
- T-cell receptor β-subunits bind to viral proteins gp120 and gp160, leading to a cytotoxic effect and membrane damage.
By what mechanisms can a complex virion enter a host cell after adsorption?
A complex virion possessing an envelope can enter the host cell via fusion of the viral envelope with the host cell's plasma membrane.
This process is described for the human immunodeficiency virus:
- the mechanism involves the fusion of the viral envelope with the target cell plasmalemma;
- as a result, the viral genomic RNA and enzymes, notably reverse transcriptase, are released into the cytoplasm.
For simple non-enveloped viruses, receptor-mediated endocytosis and pore formation are described separately in literature; fusion mechanisms are not characteristic of them.
Class and exam questions
Can a single virion contain both DNA and RNA simultaneously?
No. A fundamental property of viruses is the presence of only one type of nucleic acid—either DNA or RNA.
What is the origin of the viral envelope?
The envelope (lipoprotein membrane) is formed from fragments of the host cell's plasma membrane at the moment the virion exits the cell.
Why does a virus need reverse transcriptase?
This enzyme is required for the process of reverse transcription—creating a DNA molecule based on viral RNA inside an infected cell.
What enables viruses to crystallize?
The ability to crystallize is due to the regular spatial architecture of the capsid, which consists of repeating identical subunits (capsomeres) with helical or cubic symmetry.
What else to study on this topic
- Mechanism of virus adsorption on the host cell surface
- The role of envelope glycoproteins in receptor interaction
- Helical and cubic symmetry of capsids: examples of viruses
- The process of reverse transcription and the work of revertase
- Functions of integrase and protease in the life cycle of highly organized viruses
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