Yes, all viruses possess a capsid.
The capsid is a crucial protein shell that encases and protects the viral nucleic acid genome (DNA or RNA) from the external environment. Without a capsid, the viral genome would be vulnerable to degradation by enzymes, physical damage, and the host's immune system. The capsid is essential for the virus's survival and infectivity.
Functions of the Capsid:
- Protection: The primary role of the capsid is to shield the viral genome from damage, including UV radiation, nucleases (enzymes that degrade nucleic acids), and extreme pH levels.
- Recognition and Attachment: The capsid often contains specific proteins that allow the virus to recognize and attach to host cells. This is a critical step in the infection process.
- Delivery: In some viruses, the capsid directly participates in delivering the viral genome into the host cell, either by injecting it or by facilitating entry of the entire virus particle.
- Self Assembly: Capsids exhibit the characteristic of self-assembly, i.e. they do not need a host cell's help to construct its structure.
Capsid Structures:
Virus capsids typically exhibit two main structural shapes:
- Helical: These capsids are shaped like rods or filaments, with the protein subunits arranged in a spiral fashion around the nucleic acid. An example is the Tobacco Mosaic Virus.
- Icosahedral: These capsids are spherical or polyhedral, with 20 triangular faces. An example is the Adenovirus.
Some viruses have more complex capsid structures that don't fit neatly into either the helical or icosahedral category.
In summary, the capsid is an indispensable component of every virus, providing protection for the genetic material and facilitating infection of host cells.
