Viral Diversity & Infections

Viral Morphology and Diversity

• Viruses: infectious particles made up of nucleic acid and a protein coat
• Extremely small and simple particles
• Not exactly living, but not simply a chemical
• Have variable types of nucleic acid
• Double-stranded DNA
• Single-stranded DNA
• Double-stranded RNA
• Single-stranded RNA
• Viruses are named after the type of nucleic acid (DNA virus or RNA virus)
• Some viruses have as few as 4 genes others have hundreds
• Capsid: protein shell around nucleic acid
• Rod-shaped Example: Tobacco Mosaic Virus
• Polyhedral Example: Adenovirus
• Complex Examples: Influenza and Bacteriophage

Viral Infections

• Viruses can not replicate without infecting hosts

• Host: the organism infected with the virus
• Host Range: number of different species a virus can infect
• Most viruses only infect a single host species (specialist) Example: Measels Virus only infects humans
• Some viruses have a wide range of hosts (generalist) Example: West-Nile Virus can infect Mosquitos, Birds, Humans etc.

Viral Replication

• Virus attaches to the outside of host cell
• Virus injects nucleic acid into host
• Virus hijacks the cell machinery to replicate its own components
• Viral components come together to form new viruses
• Virus is ejected from cell to infect other hosts
• Lytic Cycle: the final step in viral replication bursts open and kills host cell
• These types of viruses are virulent phages
• Lysogenic Cycle: viruses do not kill host cell
• Virus genes continue to be produced by host cell
• Temperate phages alternate between lytic and lysogenic cycles