Virus Structure

Early Virus Discovery

Pasteur Developed Vaccine for Rabies - 1884

• Grew Virus in Rabbits → Dried Nerve Tissue to Weaken Virus

  • Attenuation → Weaken / Reduce Pathogen

• Coined Term Virus → Latin for Poison

Filterable Virus

Infectious Fluids → Passed Through Porcelain Filters

• Cell-Free Liquid was still Infectious

Proved agents smaller than Bacteria causes Disease

Unique Properties of Viruses

Infectious Particles

• Instead of Organism

Active or Inactive

• Instead of Alive / Dead

Obligate Intracellular Parasites

• Cannot Multiply unless in Host

• Instruct Genetic Pathways of Host to make new Virus

• Some Bacteria are Smaller than Intracellular Parasites

General Characteristics

Virion → Complete, Fully Developed Infectious Viral Particle

Single type of Nucleic Acid

• DNA or RNA

Protein Coat surrounding Nucleic Acid

• Capsid

Inactive Macromolecules outside Host

Host Range

Some Infect Invertebrates, Vertebrates, Plants, Protist, Fungi, Bacteria

Most can only infect Specific Types of Cells of one Host Species

Determined by

• Requirement for Attachment

• Receptors

• Availability of Cellular Factors

Virus Size

Measured in Nanometers

20 nm to 1000 nm Diameter

Electron Microscopes are Required

Parvoviruses → 0.02 um in Diameter

Mimiviruses → 450 nm, Larger than some Small Bacteria

Pandoravirus → 1 um Long in Amoeba

Pandora > Mimi > Parvo

Viral Capsid + Envelope

Capsid → Protein Coating Surrounding Nucleic Acid

• Subunits called Capsomeres

  • Some Single Protein, some Mosaic

• Variety of Shapes

Envelope → Found in SOME Viruses

• Lipid Bilayer with Protein and Carbohydrates SURROUDING Capsid

  • Part of Membrane from Host Cell

Viruses without Envelopes → Naked

Spikes → Glycoproteins

• Some Viruses

• Project from the Envelope or Capsid

• Attach Virions to Receptors

• Change Composition to evade Immune System

Morpholohy

Based on Capsid Structure

Helical → Long Rods

• Ebola + Rabies
  
  

Polyhedral / Icosahedral → Many Sided

• Icosahedron → 20 Triangle Faces, 12 Corners

• Adenoviruses + Poliovirus

Enveloped → Either Or

• Influenzavirus + Herpes Simplex Virus

Complex → Complicated Structures

• Bacteriophage

Complex Capsid

Bacteriophages → Viruses that Infect Bacteria

Multiple Protein Types

Not Symmetrical

Viral Envelope

Composed of Membrane System of Host

• Cell Membrane or Nuclear Membrane

Regular Membrane Proteins Replaced with Viral Proteins

Spikes → Protruding Glycoproteins for Attachment

Viral Nucleic Acid Structure

DNA or RNA → Never Both

• Single or Double Stranded

  • ssDNA, dsDNA, ssRNA, dsRNA

Linear or Circular

Some Several Segments

Nucleic Acids - At the Core of a Virus

Genome → Complement of DNA or DNA

Number of Viral Genes is Small compared to Cell

Only have Genes Necessary to Invade Host

Few Thousand Nucleotides - 250k

• Handful of Genes → 10

  • Influenza A virus → 7 Genes

  • HIV → 9 Genes

  • Pandoraviruses → 2.5k Genes

Viral Nucleic Acids

Positive-Sense RNA

• Single Stranded RNA Genomes ready for Translation into Proteins

Negative-Sense RNA

• RNA Genomes that needs to be Converted to be made into Proteins

Other Substances in Virus Particles

Enzymes for Operation

• Polymerase → Synthesize DNA + RNA

• Replicases → Copy RNA

• Reverse Transcriptase → Synthesizes DNA from DNA

  • Only in Retroviruses

  • Unique to Certain Viruses → Not made by Eukaryotes / Bacteria

Substances from Host Cell

• Arenaviruses pack Along Host Ribosomes

• Retroviruses “borrow” Host tRNA molecules