Viruses and Infectious Agents

Virus

• Submicroscopic, infectious agent containing genetic material (DNA or RNA).
• Requires a living cell to replicate.
• Can be toxic (e.g., Hepadna & Retro viruses).

Virion

• Complete, mature, and infectious virus particle.
• Consists of a single molecule of nucleic acid surrounded by a protein coat (capsid).
• Simplest viruses have a viral genome (core) and a viral protein coat.

Viroid

• Small, self-replicating, single-stranded circular RNA molecule.
• Infects only plants.
• Subviral agent.

Virusoid

• Type of viroid that needs a "helper virus" to complete its life cycle.
• Helper virus provides the capsid for the virusoid.

Prion

• Infectious protein without nucleic acid.
• Example: Mad cow disease.

Facts About Viruses

• Viruses can infect all types of life, including animals, plants, bacteria, and protozoa.
• Outside of a cell, viruses are metabolically inert.
• Viruses replicate their genome by replicating the nucleic acid, not by binary fission.
• Viruses lack a cell wall and a complete enzymatic system.
• They are resistant to antibiotics (except for some minor exceptions like Neomycin).
• Antivirals and cellular glycoproteins (interferons) have significant effects on viruses.

Differences Between Viruses and Other Microorganisms

Virus Structure

• Composed of two main components:

  • Genetic material (genome): either DNA or RNA, never both.
  • Capsid: the protein coat surrounding the genome.

• Viral envelope: Not always present, can be found on some viruses.

  • Viruses without an envelope are called naked viruses.
  • Viruses with an envelope are called enveloped viruses.

Virus Genome

• A giant thread-like molecule composed of polymers of genes.

• Each gene is composed of smaller structural units called nucleotides.

• Nucleotide structure:

  • Pentose sugar: Ribose in RNA viruses, Deoxyribose in DNA viruses.
  • Nitrogenous base: Purines (Adenine and Guanine), Pyrimidines (Thymine, Cytosine, and Uracil).

• Location of viral genome:

  • Intranuclear: typically found within the nucleus of the infected cell.
  • Intracytoplasmic: Found in the cytoplasm of the infected cell (e.g., HIV).

• Functions of the viral genome:

  • Carries genetic information of the virus.
  • Redirects cellular metabolic machinery towards the synthesis of new viral organelles.

• Genome Size:

  • Smallest DNA virus: Chicken infectious anemia virus 1.7-1.8 kb.
  • Smallest RNA virus: Hepatitis D virus 1.8 kb.
  • Largest DNA virus: Pandora virus 2.5 million base pairs (Mbp) and Pox virus 135-350 kbp.
  • Largest RNA virus: Coronavirus 30,000 bp.

How Viruses Compress Their Genomes?

• Strategy 1: Distribution of the virus genome on both directions of the double-stranded DNA. This leads to different proteins being produced when read in different directions (3'-5' vs 5'-3').
• Strategy 2: Overlapping of the viral genome (Open Reading Frames = ORF). Each ORF has a start and stop codon. Example: Hepatitis B virus.
• Strategy 3: mRNA segments are cut into smaller segments by mRNA splicing, creating different proteins. Example: Influenza virus.
• Strategy 4: Translation forms a very long polypeptide chain. Virus uses a protease enzyme to cleave this chain into smaller proteins. Example: Picornavirus.

Protein Coat (Viral Capsid & Viral Envelope)

A) Viral Capsid

• Definition: Structural protein produced from the viral genome.

• Characteristics:

  • Formed from capsomeres arranged in clusters by non-covalent hydrogen bonds.
  • Each capsomere is composed of smaller subunits called protomers.
  • Capsid is metastable: non-covalent bonding allows for disassembly and reassembly.
  • Function:
    • Protection of the genome: stable protective protein shell.
    • Assists in the envelopment process.
    • Fusion of the virion envelope to a cell membrane (for enveloped viruses).
    • Attachment of the virion to a host cell (for many viruses).
    • Delivery of the genome to the appropriate site.

• Symmetry or Morphology:

  • Cubic, icosahedral, geometrical, polyhedral, or isometric.

    • Icosahedral: 12 corners, 30 edges, 20 triangular faces.
    • Triangulation: Number of subunits present at each face (T1, T2, T3, T4).
    • Quasi-equivalence: Capsids with more than 60 subunits have quasi-equivalent positions.

  • Examples:

    • RNA viruses: Picornavirus, Flavivirus.
    • DNA viruses: All are icosahedral except Poxvirus.

  • Helical (spiral, screw, or tubular): Capsomers arranged spirally around a central axis.

    • Examples:
      • Animal viruses.
      • RNA viruses: All except Picornavirus, Flavivirus, Calicivirus, and Retrovirus.

B) Viral Envelope

• Characteristics:

  • Determines physical and chemical properties of the virus. Sensitive to lipid solvents (e.g., ether, chloroform). Also known as heat-labile viruses.
  • Obtained from cytoplasmic membrane, nuclear membrane, Golgi apparatus, or endoplasmic reticulum.

• Function:

  • Enveloped viruses are often more virulent than naked viruses.

C) Naked Virus

• Resistant to lipid solvents.

Chemical Structure of Virus

• Enzymes involved in virus-cell interaction: HA, NA, F.
• Enzymes involved in replication: pol. & lig.
• Enzymes providing mRNA transcription: Viral transcriptases (DNA-dependent RNA polymerase or RNA-dependent RNA polymerase).
• Enzymes inducing reverse transcription: RT (RNA-dependent DNA polymerase).
• Enzymes with specific functions: IN (Integrase).

Conclusion

This information provides a comprehensive understanding of virus structure and function. Understanding these details helps us to develop effective antiviral therapies and strategies to combat viral infections.