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Eukaryotes and Viruses. Chapters 12 and 13. Viral Characteristics and Structure. Why Viruses aren’t Alive. General Characteristics of Viruses. Name derives from the Latin for “poison” Obligatory intracellular parasites Referred to as filterable Contain a single type of nucleic material

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eukaryotes and viruses

Eukaryotes and Viruses

Chapters 12 and 13

viral characteristics and structure
Viral Characteristics and Structure

Why Viruses aren’t Alive

general characteristics of viruses
General Characteristics of Viruses
  • Name derives from the Latin for “poison”
  • Obligatory intracellular parasites
  • Referred to as filterable
  • Contain a single type of nucleic material
  • The nucleic material is covered in a protein coat.
  • Use the synthesis machinery of the host to multiply.
why are they not alive
Why are they not Alive?
  • No independent metabolism or reproduction
  • No single phylogenetic origin
  • No cellular structure
  • No ribosomes
  • Though they DO evolve and reproduce.
host range
Host Range
  • Viruses have a specific subset of cell types they will infect, referred to as Host Range.
  • Most viruses can only infect a single species
  • Some viruses can cross species barriers
  • Numerous factors influence host range
  • Viruses that infect bacteria are referred to as bacteriophage or simply phage.
viral particle size
Viral Particle Size

Staphylococcus Bacteria

1 μm in diameter





viral structure
Viral Structure
  • A Virionis a complete, infectious viral particle and is composed of…
    • Nucleic Acid
    • Capsid and Envelope
nucleic acid
Nucleic Acid
  • Only a single type of nucleic acid (RNA or DNA) is present in any species of virus.
  • Unlike cellular life, viral nucleic acid can be either single or double-stranded (again only a single type per species)
  • Size of the genetic structure can range from a few thousand base pairs to a quarter of a million
capsid and envelope
Capsid and Envelope
  • Capsids are regular repeating protein structures composed of capsomeres.
  • Some viral species also have a host-derived envelope surrounding the capsid
  • Some viral species have protein/ carbohydrate “spikes” rising from the surface that can be used for identification
viral taxonomy
Viral Taxonomy

How do you do a taxonomy of something that isn’t alive?

viral taxonomy1
Viral Taxonomy
  • Without a shared phylogeny there is no use for the higher taxons (Domain, Kingdom, Phylum, and Class)
  • Typically viral species are referred to by Order, Family, Genus and a descriptive common name (in place of a species epithet)
  • Based on
    • Nucleic Acid type
    • Strategy of replication
    • Morphology
    • Host range
viral replication
Viral Replication

No, not 1 becomes 2, more like 1 becomes 1000.

viral identification
Viral Identification
  • Polyphasic Identification
    • Morphology
    • Detection of Antibodies
    • Western Blotting of known viral proteins
    • Nucleic Methodologies
      • PCR
      • RFLP
      • RNA PCR
lytic bacteriophage cycle
Lytic Bacteriophage Cycle






dna vs rna viral replication
DNA vs. RNA Viral Replication
  • There are various types of RNA viruses.
  • Replication of the Genetic Material can be simple or a multistep process.
    • +RNA, direct translation and replication by viral protein
    • -RNA, indirect translation and replication by viral protein
    • dsRNA, direct translation and relication by viral protein
    • Retroviruses, conversion of RNA to DNA, integration and then production by host.



Attachment to plasma membrane proteins and glycoproteins.

Capsid enters cells

Capsid removed by enzymes

Biosynthesis in nucleus or cytoplasm


Enveloped viruses bud and nonenveloped rupture.

  • Attachment to Cell Wall proteins.
  • Viral DNA is injected into cell
  • No removal of capsid required
  • Biosynthesis in cytoplasm
  • Lysogeny
  • Host cell lysed for release
viruses and cancer
Viruses and Cancer

One of many factors

oncogenic viruses
Oncogenic Viruses
  • Some viruses and known to help trigger cancers, called oncogenic viruses.
  • These viruses affect oncogenes, natural parts of our genetic structure that can cause cancer.
  • The process of becoming cancerous is termed transformation.
  • Oncogenic Viruses integrate into the host genetic material.
acute latent and persistent viral infections
Acute, Latent, and Persistent Viral Infections
  • Acute Infections are those that cause immediate proliferation.
  • Latent infections can occur by itself or after an acute infection, where the viral load remains undetected for a long period of time before reemerging quickly.
  • Persistent Infections are ones where the viral load build over a long period of time.
  • Prions are infectious protein particles
  • Prions are altered forms of a normal protein in the host that can catalyze the alteration of other “normal” protein particles to the “prion” state
  • They cause neurological degradation and death with no known treatment.
  • Since each prion protein is infectious, they are extremely resistant to control measures.
prion reaction
Prion Reaction

PrPC + PrPSc 2 PrPSc