Study Notes: Viruses, Viroids, and Prions (Chapter 13)

Viruses, Viroids, and Prions

General Characteristics and Morphology of Viruses

Viruses are unique infectious agents that differ fundamentally from cellular life forms. They possess distinct structural and physiological features.

• Not Cells: Viruses lack a cell membrane and cellular organelles. They are not considered living cells.

• Structure: A virus consists of a chromosome (nucleic acid), a capsid (protein coat), and, in some cases, an envelope (phospholipid bilayer with viral spike proteins).

• No Ribosomes: Viruses do not have ribosomes and cannot carry out protein synthesis independently.

• Limited Metabolism: Viruses rely entirely on the host cell for energy, amino acids, nucleotides, and other metabolic functions.

• Assembly, Not Division: Viruses do not divide; instead, their components are synthesized separately and then assembled into mature particles (virions).

• Single Nucleic Acid Type: Each virion contains only one type of nucleic acid (either DNA or RNA), unlike cells which have both.

General Physiology of Viruses

Viruses parasitize host cells, using their machinery to produce viral proteins and enzymes.

• Host Cell Contributions: The host provides replication enzymes, transcription enzymes, translation apparatus, and metabolic energy (ATP).

• Viral Assembly: Viral components (chromosome and capsid proteins) are synthesized independently and then assembled into virions.

• Difference from Cells: Cells reproduce by division (1 to 2 cells), while viruses assemble from synthesized parts.

Viral Life Cycle

The viral life cycle consists of several distinct stages, each critical for successful infection and replication.

1. Attachment: Viruses bind specifically to host cell receptors. This specificity can be tissue-specific or species-specific. Some viruses (e.g., Rabies, West Nile virus, Ebola) have broader host ranges.

2. Penetration (Entry): Viruses enter host cells by injecting their chromosome (bacteriophage), fusion of envelope with cell membrane (enveloped animal viruses), or endocytosis.

3. Uncoating: The capsid is removed, exposing the viral nucleic acid for replication.

4. Synthesis: Viral proteins and nucleic acids are synthesized in an orderly manner. Coat proteins are typically synthesized last.

5. Assembly (Maturation): Viral components spontaneously assemble into mature virions when sufficient concentrations are reached.

6. Release: Viruses exit the host cell by lysis (breaking open the cell) or budding (pinching off with an envelope).

Synthesis Details for Selected Viruses

• dsDNA Viruses:

  • Host RNA polymerase transcribes viral DNA.

  • Host ribosomes translate early proteins.

  • Viral DNA is replicated.

  • Late transcription and translation produce capsid proteins.

• +RNA Viruses:

  • +RNA acts as mRNA; host ribosomes translate early proteins.

  • Early protein: RNA-dependent RNA polymerase.

  • This enzyme synthesizes complementary -RNA, which serves as a template for more +RNA.

  • +RNA is used for late gene translation and assembly.

• -RNA Viruses:

  • -RNA is the complement to mRNA; must bring RNA-dependent RNA polymerase into the host.

  • This enzyme synthesizes +RNA from -RNA.

  • +RNA is translated by host ribosomes and transcribed by viral polymerase.

• Retroviruses (e.g., HIV):

  • +RNA chromosome, but replication is unique.

  • Bring pre-formed enzymes: reverse transcriptase, integrase, protease.

  • Reverse transcriptase synthesizes complementary DNA from RNA and degrades RNA.

  • dsDNA integrates into host chromosome via integrase; host RNA polymerase transcribes viral genes.

  • Host ribosome translates +RNA into a long polypeptide; protease cleaves it into separate viral proteins.

Example: Protease inhibitors, introduced in the mid-1990s, revolutionized HIV therapy by blocking the cleavage of viral polypeptides, halting viral replication.

Viruses and Cancer

Some viruses are oncogenic, meaning they can cause cancer by disrupting normal cell regulation.

• HPV and Cervical Cancer: Human papilloma virus (HPV) is a DNA virus responsible for most cervical cancers. Vaccines have been developed to prevent infection.

• Retroviruses: Some retroviruses carry viral oncogenes (vONC), analogs of cellular oncogenes (cONC). Insertion of vONC into host DNA can lead to uncontrolled cell division and metastasis.

• Mechanism: Oncogenic viruses disturb the balance of cell regulation genes, promoting excessive cell proliferation and viral production.

Prions

Prions are infectious proteins that cause neurodegenerative diseases in animals.

• Definition: Prions are misfolded forms of normal host glycoproteins.

• Diseases: Prion diseases are progressive, fatal, and affect the nervous system.

• Examples: Creutzfeldt-Jacob disease, Bovine Spongiform Encephalopathy (Mad cow disease), Scrapies in sheep.

• Transmission: Prion diseases can cross species barriers, as seen in BSE transmission from cows to humans.

Viroids

Viroids are small, circular RNA molecules that infect plants.

• Structure: Viroids are 300-400 nucleotides long, too short to encode proteins.

• Transmission: Spread by insects or human activity, causing severe plant diseases.

• Impact: Viroids can devastate crops, affecting agricultural productivity.

Coat Protein Mediated Resistance

Biotechnological advances have enabled the engineering of plants and animals to resist viral infections by expressing viral coat proteins.

• Mechanism: Expression of viral capsid protein in host cells prevents viral uncoating, blocking infection.

• Example: Grapevine fan leaf virus resistance in Chardonnay grape plants; engineered chickens resistant to Avian leukosis virus.

• Benefits: Reduces need for insecticides and enhances agricultural productivity.

Summary Table: Types of Viral Genomes and Replication Strategies

Virus Type	Genome	Key Enzyme	Replication Strategy	Example
dsDNA Virus	Double-stranded DNA	Host RNA polymerase	Transcription, translation, DNA replication	Herpesvirus
+RNA Virus	Single-stranded RNA (+)	RNA-dependent RNA polymerase	Translation, synthesis of -RNA, then +RNA	Poliovirus
-RNA Virus	Single-stranded RNA (-)	Viral RNA-dependent RNA polymerase	Synthesis of +RNA, translation	Influenza virus
Retrovirus	Single-stranded RNA (+)	Reverse transcriptase, integrase, protease	Reverse transcription to DNA, integration, transcription, translation	HIV
Viroid	Small circular RNA	None	Direct interference with host gene expression	Potato spindle tuber viroid
Prion	Protein (no nucleic acid)	None	Induces misfolding of host proteins	Creutzfeldt-Jacob disease

Additional info: The table above summarizes the main types of viral genomes and their replication strategies, including examples for each type.