Virus Structure and Genomes

Virus Structure and Shapes

• The complete, mature, and infectious viral particle is known as a virion. A typical virion is composed of the following parts:
• Outer Coat:
  • Capsids (inner core)
  • Envelope (tail fibers/spikes)
• Inner Core:
  • DNA or RNA (genome)
  • Reverse transcriptase (in some cases)

Genome

• The genome is the total genetic material, also called the nucleoid
• It's made up of single or many DNA or RNA molecules
• Smaller viruses have one genome molecule, larger viruses have several hundred
• Viral proteins are also present, which have an interactive role with host cell components
• Genomes encode genetic information for replication, transcription, translation, viral assembly, viral release, host-virus interactions, and immune evasion
• Types of genomes include DNA, RNA, and reverse transcriptase

Capsid

• The capsid is the protein coat surrounding the genome
• It's made up of protein subunits called capsomers.
• Capsomer shapes vary among different viruses (e.g., Adenovirus - 252, Rhinovirus - 180, Poliovirus - 180, Rotavirus - 200, Hepatitis virus - 120, HIV - 200, Bacteriophage T4 - 115, TMV - 2130)
• Variations in capsids include icosahedral (20 sided), helical (cylindrical), complex (combination of icosahedral and helical), flexible, pleomorphic (irregular), variable shapes (spherical, prolate (elongated), oval, shape, oblate (flattened), click-like shape), tadpole (oval shaped head with tail), polyhedral, etc.

Functions of Capsid

• Stability, protection, attachment, penetration, assembly, and fusion with host membrane
• Some viruses have accessory structures called viral envelopes (or cloaking or covering), classifying them as enveloped or unenveloped viruses.
• Enveloped viruses have a host-derived membrane composed of a lipid bilayer and glycoprotein spikes, envelope proteins and surface antigens facilitating attachment to host cells, entry into host cells, fusion with host membranes, immune evasion, viral assembly, viral release by budding

Tail Fibers/Spikes

• These are additional fibers
• Some viruses have tail fibers
• These are long protein structures extending from the viral tail
• Composed of fibrous protein (like collagen fibers)
• Short, spike-like protein structures that are located on the viral tail or surface
• Composed of glycoprotein
• Functions of tail fibers/spikes:
  • Host Recognition
  • Attachment to host cells
  • Adsorption to host cells (initial binding to the host)
  • Penetration to host cells
  • Genome delivery