Chapter 6 Viruses Overview

Acellular Pathogens: Viruses

• Definition: Viruses are submicroscopic, infectious, parasitic, non-living agents. They consist of nucleic acid (DNA or RNA, single or double-stranded, circular or linear) surrounded by a protein coat (capsid).

Capsid Structures

• Forms: Capsids have three basic shapes: helical, icosahedral, and complex (a combination of the first two).

Host Range and Specificity

• Host range: The range of organisms a virus can infect.
• Tissue specificity: Within a host organism, viruses infect specific tissues or organs.
• Bacteriophages: Infect specific bacterial species.
• Smallpox: Infects only humans.
• Classification criteria: Host organism, morphology (capsid structure), and genome type (DNA or RNA).

Viral RNA Types

• +ssRNA: Contains the code for protein synthesis and can be directly translated by ribosomes.
• -ssRNA: Requires an intermediate step; it serves as a template to create +ssRNA, which is then used for protein synthesis.

Bacteriophages

• Definition: A bacteriophage is a virus that infects and replicates within a bacterial cell.
• Replication steps:
  • Attachment: Phage attaches to the bacterial host cell.
  • Penetration: Viral DNA enters the host cell (but the capsid does not enter the cytoplasm).
  • Biosynthesis: Phage DNA is replicated and phage proteins are produced, hijacking the host cell's machinery.
  • Maturation: New phage particles are assembled.
  • Lysis: The host cell is broken open, releasing newly formed phages.

Phage Types

• Virulent phages: Undergo the lytic cycle, resulting in the death of the host cell.

• Temperate phages: Undergo the lysogenic cycle, integrating their DNA into the host cell's genome, replicating with the host until stress triggers the lytic cycle.

• Lytic cycle trigger: Stress induces temperate phages to switch to the lytic cycle.

Animal Virus Replication

• Differences from bacteriophages: Replicates differently inside animal host cells compared to bacteria.
• Uncoating: The process where the viral nucleic acid is released from the capsid.

Viral Penetration

• Enveloped viruses: Fusing with the host cell's plasma membrane, releasing the nucleocapsid into the cytoplasm.
• Non-enveloped viruses: Entering the cell by endocytosis (engulfment).

Viral RNA Replication

+ssRNA - Codes for protein synthesis directly on its own.

• ssRNA - Creates a +ssRNA template to provide the code for protein synthesis.

Viral Release

• Enveloped viruses: Release by pushing through the host cell membrane, acquiring an envelope in the process, or budding (fusing with the plasma membrane and releasing the virus).

Retroviruses

• Definition: An RNA virus.
• Key enzyme: Reverse transcriptase - converts their RNA into DNA.

Viral Growth Curve

• Steps: Includes Inoculation, Eclipse, Burst phases and defining Burst size - maximum virions per bacterium.

Animal Virus Cultivation

• Techniques:
  • Fertilized chicken eggs: Used for growing animal viruses (amniotic cavity, chorioallantoic membrane, yolk sac).
  • Cell cultures:
    • Primary cell cultures: Isolated from animal tissues, limited lifespan due to contact inhibition.
    • Secondary cell cultures: Derived from primary cultures, increased lifespan.
    • Continuous cell lines: Derived from transformed cells/tumors, unlimited growth potential, unresponsive to contact inhibition.

Bacteriophage Cultivation

• Method: Cultivation on a 'bacterial lawn'.

Plaque Formation

• Definition: Clear zones on a bacterial lawn indicating areas where bacteriophages have lysed bacterial cells.

Virus Detection

• Methods: Observing cytopathic effects (structural changes) under a microscope (e.g. syncytia, cell inclusions).

Viroids

• Definition: Non-infectious, single-stranded RNA molecules causing disease in plants.
• Characteristics: Need a host cell to replicate, do not encode proteins.
• Examples: Potato spindle tuber viroid (PSTV), tomato planta macho viroid (TPMVD), Avocado sunblotch viroid (ASBVD)

Virusoids

• Definition: Infectious, single-stranded RNA molecules, needing a helper virus to replicate.
• Characteristics: Do not encode proteins
• Examples: Subterranean clover mottle virus, Hepatitis B virus

Prions

• Definition: Infectious, self-replicating proteins causing diseases.

• Mechanism: Caused by genetic mutation or spontaneous conversion.

• Types of prion proteins:

• PrPc: Normal cellular prion protein.

• PrPsc: Abnormal, misfolded prion protein; causes prion diseases.

• Transmission: PrPSC converts normal PrPc to abnormal PrPsc through the formation of heterodimers. The abnormal proteins accumulate causing disease.

Antiviral Drugs

• Targets: Various stages of viral replication, including penetration/uncoating, genome replication, and virion maturation/release.
• Examples of antiviral drugs : (listed by function, not by drug name)
• Blocking Penetration/Uncoating: Amantadine, Rimantadine, Pleconaril, Envufirtide, Maraviroc
• Inhibition of Genome Replication: Acyclovir, Ganciclovir, Ribavirin, AZT, Vidarabine , Idoxuridine, Trifluridine, Dideoxycytidine, Dideoxyinosine, Foscarnet, Nevirapine, Delavirdine, Raltegravir
• Inhibition of Virion Maturation/Release: Indinavir, Ritonavir, Saquinavir, Oseltamivir, Zanamivir

Explore the fascinating world of viruses, including their structures, host range, and classification. This quiz covers the essential aspects of viral morphology, RNA types, and their specificity. Test your knowledge of these acellular pathogens and their roles in infections.