Viruses: Structure, Replication, and Other Infectious Agents

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 and immune evasion.

• Capsid: The protein shell that encloses the viral genome. Capsids can have various shapes and are 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 bilayer derived from the host cell membrane, found in many animal viruses. It contains viral proteins and helps the virus evade the host immune system.

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

Example:

• Diagram of a bacteriophage showing DNA, protein capsid, and tail fibers.

• Diagram of an enveloped animal virus showing glycoprotein spikes, genome, capsid, and envelope.

Virus Replication

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

• Entry: Viruses may inject their genome into the host (e.g., bacteriophages), be absorbed by endocytosis, or fuse with the host membrane.

• Viral Replicator: The virus uses the host's cellular machinery to produce viral components, including nucleic acids and proteins.

Example:

• Diagram showing bacteriophage injecting DNA into a bacterial cell.

• Diagram of animal virus replication, including entry, uncoating, replication, and assembly.

Lytic and Lysogenic Cycles

Bacteriophages can replicate via two main cycles: the lytic cycle and the lysogenic cycle.

• Lytic Cycle: The phage injects its DNA, which is replicated and used to synthesize viral components. The host cell is lysed, releasing new viruses.

• Lysogenic Cycle: The phage DNA integrates into the host genome and is replicated along with it. The viral DNA (prophage) can later exit the genome and enter the lytic cycle.

Example:

• Flowchart illustrating the steps of the lytic and lysogenic cycles.

Animal Virus Replication

Animal viruses often have enveloped structures and RNA genomes. Their replication involves entry via cell surface proteins and synthesis using host machinery.

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

• Reverse Transcriptase: The enzyme that catalyzes the formation of DNA from an RNA template.

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

Example:

• Diagram of retrovirus replication, showing reverse transcription and integration into host DNA.

Virus Genomes

Viruses can have different types of genomes, which affect their replication strategies.

• Double-stranded DNA viruses: Replicate their DNA in the host nucleus and use host enzymes for transcription and replication. Infect a wide range of organisms.

• Double-stranded RNA viruses: Use viral RNA polymerases for replication and transcription.

• Positive-sense RNA viruses: Their genome can be directly translated into viral proteins by host ribosomes.

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

• Retroviruses: Use reverse transcriptase to produce DNA from their RNA genome.

Example:

• Diagram showing transcription of viral DNA and RNA into mRNA.

Other Infectious Agents

Some infectious agents are even simpler than viruses and can cause disease in plants and animals.

• 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 abnormal folding of normal proteins, leading to disease.

Example:

• Diagram of viroid structure and replication.

• Diagram showing prion propagation and conversion of normal proteins to prion form.

Summary Table: Virus Types and Replication Strategies

Type	Genome	Replication Strategy	Example
Double-stranded DNA virus	dsDNA	Replicates in host nucleus, uses host enzymes	Herpesvirus
Double-stranded RNA virus	dsRNA	Uses viral RNA polymerase	Rotavirus
Positive-sense RNA virus	+ssRNA	Genome directly translated	Poliovirus
Negative-sense RNA virus	-ssRNA	Transcribed to mRNA by viral polymerase	Influenza virus
Retrovirus	ssRNA	Reverse transcription to DNA, integration	HIV
Viroid	ssRNA	Replicates using host enzymes	Potato spindle tuber viroid
Prion	Protein	Induces abnormal folding of host proteins	Mad cow disease

Key Equations

• Transcription of DNA to mRNA: $\text{DNA} \xrightarrow{\text{RNA polymerase}} \text{mRNA}$

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

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.