Viruses

Virus Structure

Viruses are acellular infectious agents that are much smaller than cells and require host cells for replication. They consist of genetic material encased in a protein shell, and sometimes additional structures.

• Capsid: The protein coat that surrounds and protects the viral genome. Capsids can have various shapes (helical, icosahedral, complex).

• Capsomere: The subunit of the capsid.

• Viral genome: Can be double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA.

• Bacteriophages: Viruses that infect bacteria, often with complex capsids.

• Viral envelope: A lipid bilayer derived from the host cell membrane, present in many animal viruses. It contains viral proteins and glycoproteins.

• Host range: The spectrum of host cells a virus can infect, determined by specific interactions between viral surface proteins and host cell receptors.

Example: A bacteriophage with a complex capsid structure and a typical animal virus with an envelope and glycoproteins.

Virus Replication

Viral replication begins when the virus binds to the host cell and introduces its genome. The replication strategy depends on the type of virus.

• Some viruses, like bacteriophages, inject their genome into the host cell.

• Some animal viruses enter by endocytosis or membrane fusion.

• Once inside, the viral genome hijacks the host's machinery to produce viral components.

• Viral replicase: Enzyme that replicates the viral genome using host resources.

Example: Diagram showing viral entry and replication inside a host cell.

Lytic and Lysogenic Cycles

Bacteriophages can follow two main replication cycles: lytic and lysogenic.

• Lytic cycle: The phage replicates rapidly, leading to host cell lysis and release of new viruses.

  • Phage injects DNA, host DNA is degraded, viral components are synthesized and assembled.

  • Virulent phage: Only replicates via the lytic cycle.

• Lysogenic cycle: The phage genome integrates into the host genome and replicates along with it without killing the host.

  • Phage DNA is called a prophage when integrated into the bacterial chromosome.

  • Phage can switch from lysogenic to lytic cycle under certain conditions.

Example: Flowchart comparing the lytic and lysogenic cycles.

Animal Virus Replication

Animal viruses often have envelopes and RNA genomes. Their replication involves entry via endocytosis or membrane fusion, and genome replication using host or viral enzymes.

• Viral RNA serves as a template for synthesis and replication by viral RNA polymerase.

• Retroviruses: RNA viruses that use reverse transcriptase to synthesize DNA from their RNA genome, which then integrates into the host genome as a provirus.

• Reverse transcriptase: Enzyme that catalyzes RNA → DNA transcription.

Example: Diagram of HIV replication cycle, showing reverse transcription and integration.

Virus Genomes and Replication Strategies

Viruses can have DNA or RNA genomes, which determine their replication strategy.

• Double-stranded DNA viruses: Use host nucleus for replication and transcription. Examples include bacteriophages and animal viruses.

• Double-stranded RNA viruses: Use viral enzymes to replicate their genome in the cytoplasm.

• Positive-sense RNA viruses: Genome acts as mRNA and is directly translated by host ribosomes.

• Negative-sense RNA viruses: Genome is complementary to mRNA; must be transcribed into mRNA by viral RNA polymerase before translation.

• Retroviruses: Use reverse transcriptase to convert RNA genome into DNA, which integrates into the host genome.

Example: Diagram showing the flow from viral genome to mRNA for different virus types.

Other Infectious Agents: Viroids and Prions

Some infectious agents are even simpler than viruses.

• Viroids: Smallest known pathogens, consisting of short, circular, single-stranded RNA. They infect plants and do not encode proteins, but replicate using host enzymes.

• Prions: Infectious proteins that cause neurodegenerative diseases in animals. Prions induce abnormal folding of normal proteins, leading to disease.

Example: Diagram showing prion-induced conversion of normal proteins into infectious prion forms.

Summary Table: Virus Types and Replication Strategies