Viruses KarinaC

Viruses

Virus Structure

Viruses are microscopic infectious agents that require a host cell to replicate. They are much smaller than cells and consist of genetic material encased in a protein shell. Some viruses also possess additional structures that aid in infection.

• Capsid: The protein shell that encloses the viral genome. It can have various shapes and is composed of subunits called capsomers.

• Genome: Viruses may contain 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 membrane derived from the host cell, found in many animal viruses, and contains proteins that help the virus enter host cells.

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

Example: A bacteriophage with a complex capsid and tail fibers for host recognition.

Virus Replication

Viral replication begins when the virus binds to the host cell and introduces its genome. The process varies among viruses:

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

• Other viruses are absorbed into the host by endocytosis or fuse their membranes with the host's membrane.

• Viral replicator: The virus uses the host's machinery to produce viral components, including nucleic acids and proteins, which are then assembled into new viruses.

Example: A virus entering a host cell and hijacking the cell's replication machinery.

Lytic and Lysogenic Cycles

Viruses, especially bacteriophages, can replicate via two main cycles:

• Lytic cycle: The virus replicates and causes the destruction of the host cell. Viral DNA is degraded, and new viral components are synthesized and assembled. The cell eventually bursts, releasing new viruses.

• Lysogenic cycle: The viral genome integrates into the host's DNA and replicates along with it. The virus remains dormant until triggered to enter the lytic cycle.

Example: Diagram showing the steps of the lytic and lysogenic cycles.

Animal Virus Replication

Animal viruses often have viral envelopes and RNA genomes. Their replication involves entry into the host cell via surface protein-receptor recognition, followed by synthesis and assembly of viral components.

• Retroviruses: RNA viruses that use reverse transcriptase to convert their RNA genome into DNA, which integrates into the host genome.

• Reverse transcriptase: The enzyme that catalyzes RNA to DNA transcription.

• Provirus: The integrated viral DNA within the host genome.

Example: HIV replication cycle involving reverse transcription and integration.

Virus Genomes and Replication Strategies

Viruses are classified by their genome type and replication strategy:

• Double-stranded DNA viruses: Replicate their DNA in the host nucleus and produce mRNA for protein synthesis.

• Double-stranded RNA viruses: Use viral RNA polymerase to replicate their genome and synthesize mRNA.

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

• Negative-sense RNA viruses: Their genome is complementary to mRNA and must be transcribed into mRNA by viral RNA polymerase.

• Retroviruses: Use reverse transcriptase to convert RNA into DNA, which is then transcribed into mRNA.

Example: Diagram showing mRNA synthesis from different viral genome types.

Other Infectious Agents

Some infectious agents are even simpler than viruses:

• Viroids: Small, circular, single-stranded RNA molecules that infect plants. They do not encode proteins but replicate using host enzymes.

• Prions: Infectious proteins that cause neurodegenerative diseases in animals. Prions can induce normal proteins to misfold, leading to disease.

Example: Diagram showing prion propagation and viroid structure.

Summary Table: Virus Types and Replication Strategies

Virus Type	Genome	Replication Strategy
Double-stranded DNA virus	dsDNA	Replicates in host nucleus, uses host enzymes
Double-stranded RNA virus	dsRNA	Uses viral RNA polymerase for replication and mRNA synthesis
Positive-sense RNA virus	+ssRNA	Genome acts as mRNA, translated by host ribosomes
Negative-sense RNA virus	-ssRNA	Genome transcribed into mRNA by viral RNA polymerase
Retrovirus	ssRNA	Reverse transcription to DNA, integration into host genome
Viroid	ssRNA (circular)	Replicates using host enzymes, no protein coding
Prion	Protein	Induces misfolding of normal proteins

Key Equations

• Central Dogma (for viruses): $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• Reverse Transcription (Retroviruses): $\text{RNA} \xrightarrow{\text{Reverse Transcriptase}} \text{DNA} \rightarrow \text{mRNA} \rightarrow \text{Protein}$

Additional info: These notes expand on the provided diagrams and brief points to offer a comprehensive overview of viral biology, suitable for General Biology students.