Viruses are fascinating microorganisms that blur the line between living and non-living. These tiny parasites hijack host cells to replicate, causing a wide range of diseases. From the common cold to deadly pandemics, viruses shape our world in profound ways.

Understanding viral structure and life cycles is crucial for developing treatments and vaccines. By exploring how viruses enter cells, replicate, and spread, we can better combat these microscopic invaders and protect public health.

Viral Characteristics and Structure

Characteristics of viral pathogens

• Non-living obligate intracellular parasites lack metabolic enzymes and ribosomes for protein synthesis rely on host cell machinery for replication (influenza virus)
• Consist of nucleic acid (DNA or RNA) surrounded by a protein coat (capsid) some have an additional lipid envelope derived from host cell membrane (HIV)
• Highly specific to host cell types due to receptor-mediated entry allows targeting of specific tissues or organs (hepatitis B virus)
• Capable of causing disease by disrupting host cell functions and inducing cell lysis leading to tissue damage and symptoms (measles virus)
• Exhibit viral tropism, which determines the specific cell types or tissues a virus can infect and replicate in

Structure of viral genomes

• Viral genomes can be DNA or RNA
  • DNA viruses: double-stranded (dsDNA) or single-stranded (ssDNA) (adenovirus)
  • RNA viruses: double-stranded (dsRNA) or single-stranded (ssRNA) (rhinovirus)
    • ssRNA can be positive-sense (+ssRNA) or negative-sense (-ssRNA)
• Genome size varies from a few thousand to hundreds of thousands of nucleotides smaller genomes often associated with higher mutation rates (influenza virus)
• Viral genomes can be linear or circular circular genomes more compact and stable (human papillomavirus)
• Some viral genomes are segmented consisting of multiple nucleic acid molecules allows for genetic reassortment and increased diversity (influenza virus)

Viral Life Cycles and Host Interactions

Stages in viral life cycles

1. Attachment: Viral surface proteins bind to specific receptors on the host cell (HIV gp120 protein binds to CD4 receptor)

2. Entry: Virus or viral genome enters the host cell through endocytosis or membrane fusion (influenza virus enters via receptor-mediated endocytosis)

3. Replication: Viral genome is replicated using host cell machinery

   • DNA viruses typically replicate in the nucleus (herpes simplex virus)
   • RNA viruses typically replicate in the cytoplasm (poliovirus)
   • Some RNA viruses undergo reverse transcription to produce DNA from their RNA genome

4. Assembly: Viral proteins and nucleic acid are assembled into new virions (hepatitis B virus capsid assembly in cytoplasm)

5. Release: Mature virions are released from the host cell through lysis or budding (influenza virus budding from host cell membrane)

Bacteriophages vs plant and animal viruses

• Bacteriophages (phages) infect bacteria
  • Lytic cycle: Phage replicates and lyses the host cell (T4 phage)
  • Lysogenic cycle: Phage genome integrates into the host genome and replicates with it (lambda phage)
• Plant viruses often transmitted by insect vectors or mechanical damage cause symptoms such as leaf mottling stunting and reduced crop yields (tobacco mosaic virus)
• Animal viruses can cause a wide range of diseases from mild to severe or fatal transmission can occur through various routes such as respiratory droplets bodily fluids or vectors (Ebola virus)

Viruses as obligate intracellular parasites

• Viruses lack the necessary components for independent replication and metabolism
• They depend on host cell machinery for:
  • Protein synthesis using host ribosomes and enzymes (influenza virus)
  • Nucleic acid replication using host polymerases and nucleotides (hepatitis C virus)
  • Energy production using host cell ATP and metabolic pathways (HIV)
• Viral replication often leads to host cell lysis or disruption of normal cellular functions causing disease symptoms (poliovirus)
• Some viruses can establish latent infections integrating their genome into the host genome without active replication allows long-term persistence in the host (herpes simplex virus)

Viral Evolution and Transmission

• Antigenic drift occurs through gradual accumulation of mutations in viral surface proteins, allowing viruses to evade host immune responses
• Zoonosis refers to the transmission of viruses from animals to humans, which can lead to the emergence of new human pathogens
• Viral vectors are modified viruses used in gene therapy and vaccine development to deliver genetic material to target cells