Viral Structure and Function

Study Notes

Viruses consist of three main components: capsid, genetic material, and sometimes a viral envelope.
The capsid is a protein shell that protects the viral genetic material. It's composed of repeating protein subunits called capsomers.
The capsid's function is to protect the viral genome (RNA or DNA), help the virus attach to host cells, and aid in releasing new viral particles.
Viral capsids can have different shapes:
- Helical (e.g., Tobacco mosaic virus) - long, cylindrical
- Icosahedral (e.g., Herpesvirus) - 20-sided, symmetrical
- Complex (e.g., Bacteriophages) - non-symmetrical
Viral genetic material carries instructions for creating new viral particles. Viruses lack machinery to replicate, relying on the host cell's mechanisms.
Genetic material can be DNA or RNA:
- DNA viruses can be single-stranded (ssDNA) or double-stranded (dsDNA). Examples include Herpesvirus (dsDNA) and parvovirus (ssDNA).
- RNA viruses can be single-stranded (ssRNA) or double-stranded (dsRNA). Examples include Influenza virus (ssRNA) and HIV (ssRNA).
- RNA viruses generally have a higher mutation rate than DNA viruses, leading to rapid evolution.

Some viruses have an outer lipid membrane called a viral envelope, derived from the host cell membrane.
The envelope contains viral proteins, often called spikes, used to attach to specific receptors on host cells.
Enveloped viruses (e.g., Influenza, HIV) are typically more fragile and susceptible to inactivation (heat, drying, detergents).
Non-enveloped viruses (e.g., Adenovirus, Poliovirus) are more stable in the environment and can survive longer outside the host.