Basic Virion Structure
The core component of any virus is the nucleocapsid, which is a combination of genetic material (nucleic acid) and a protective protein shell called the capsid. The capsid itself is composed of numerous identical subunits termed capsomeres, which are formed by polypeptide chains.
The capsid performs critical functions:
- Protection: Shields the viral genome from enzymatic degradation and adverse environmental factors.
- Receptor Function (in non-enveloped viruses): Mediates initial attachment (adsorption) to a susceptible host cell.
Enveloped and Non-Enveloped Viruses
Based on their structural features, viruses are divided into two major groups:
- Non-Enveloped Viruses: Consist solely of nucleic acid and a capsid. After replication, they typically exit the host cell by causing cell destruction (lysis). They can form crystals (e.g., foot-and-mouth disease virus).
- Enveloped Viruses: Possess an additional lipoprotein membrane surrounding the nucleocapsid, known as the envelope (or peplos).
The envelope is formed when the virion buds off from the host cell, acquiring a piece of the host membrane (plasma, nuclear, or endoplasmic reticulum membrane). This membrane typically features specialized glycoprotein spikes called peplomers. Some viruses have an additional layer beneath the envelope known as the matrix protein (M protein). Notably, enveloped viruses are sensitive to lipid solvents (e.g., ether): destruction of the lipid bilayer leads to viral inactivation.
Types of Symmetry
The overall shape of a virion largely depends on the structural arrangement of the nucleocapsid. Several types of symmetry are recognized:
- Helical: The nucleocapsid is wound into a spiral or screw-like shape (characteristic of coronaviruses and influenza viruses).
- Icosahedral (Cubic): The capsid forms an isometric hollow polyhedron enclosing the nucleic acid (e.g., herpes simplex virus).
- Complex: A combination of various structural architectures.
Proteins and Internal Structure
The internal content enclosed within the capsid is referred to as the core. Its composition varies among viruses; for instance, adenoviruses have DNA associated with histone-like proteins, while reoviruses contain inner capsid proteins.
All viral proteins are categorized into:
- Structural Proteins: Form the physical components ("body") of the virion.
- Non-Structural Proteins: Function as enzymes and regulatory factors required for viral replication inside the host cell.
While most viral proteins are encoded by the viral genome itself, virions may incorporate certain host cell proteins during assembly. Enveloped viruses also contain carbohydrates (polysaccharides) and lipids within their membranes.