Viruses: Structure, Replication, and Infectious Agents (General Biology Study Notes)

Ch. 19 - Viruses

Virus Structure

Viruses are infectious agents that are much smaller than cells and require a host to replicate. They consist of genetic material encased in a protein shell and, in some cases, additional structures.

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

• Capsomere: The protein subunit of the capsid.

• Viral genomes: May 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 membrane derived from the host cell, containing viral and host proteins, found in many animal viruses.

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

Example: Bacteriophage structure with DNA inside a protein capsid and tail fibers for host attachment.

Virus Replication

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

• Some viruses inject their genome directly (e.g., bacteriophages).

• Some enter by endocytosis or membrane fusion (common in animal viruses).

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

• The host provides nucleotides, enzymes, ribosomes, tRNAs, amino acids, and ATP for viral replication.

• Nucleic acids and proteins self-assemble into new viruses.

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

Lytic and Lysogenic Cycles

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

• Lytic cycle: Results in the destruction of the host cell.

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

  • New phages are released by lysis of the host cell.

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

  • Integrated viral DNA is called a prophage.

  • Prophage may later exit the host genome and enter the lytic cycle.

Example: Flowchart comparing lytic and lysogenic cycles.

Animal Virus Replication

Animal viruses often have envelopes and RNA genomes. Their replication involves unique steps:

• Entry via endocytosis or membrane fusion, mediated by envelope proteins.

• Viral RNA serves as mRNA or as a template for mRNA synthesis (by viral RNA polymerase).

• Retroviruses (e.g., HIV) use reverse transcriptase to convert RNA into DNA, which integrates into the host genome as a provirus.

Example: Diagram of HIV structure and replication cycle.

Virus Genomes and Classes

Viruses are classified by their genome type and replication strategy.

• Double-stranded DNA viruses: Replicate in the nucleus, use host enzymes for transcription and replication.

• Single-stranded DNA viruses: Use host enzymes to synthesize complementary DNA, then transcribe mRNA.

• Positive-sense RNA viruses: Genome acts directly as mRNA for protein synthesis.

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

• Retroviruses: Use reverse transcriptase to synthesize DNA from RNA template.

Example: Diagram showing the flow from viral genome to mRNA and protein synthesis.

Other Infectious Agents: Viroids and Prions

Some infectious agents are 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 by inducing misfolding of normal proteins.

Example: Diagram of prion propagation and viroid structure.

Summary Table: Virus Types and Replication Strategies

Virus Type	Genome	Replication Strategy	Example
Double-stranded DNA virus	dsDNA	Uses host DNA-dependent RNA polymerase for transcription	Herpesvirus
Single-stranded RNA virus (+)	ssRNA (+)	Genome serves as mRNA	Poliovirus
Single-stranded RNA virus (-)	ssRNA (-)	Viral RNA polymerase transcribes genome into mRNA	Influenza virus
Retrovirus	ssRNA (+)	Reverse transcription to DNA, integration into host genome	HIV
Viroid	ssRNA (circular)	Replicates in host cells, no protein coding	Potato spindle tuber viroid
Prion	Protein only	Induces misfolding of host proteins	Mad cow disease

Key Equations and Concepts

• Central Dogma (for viruses):

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$ $\text{(Retroviruses: RNA} \rightarrow \text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein)}$

• Reverse Transcription (Retroviruses):

$\text{RNA} \xrightarrow{\text{reverse transcriptase}} \text{DNA}$

• Viral Replication Cycle (General):

$\text{Attachment} \rightarrow \text{Entry} \rightarrow \text{Replication} \rightarrow \text{Assembly} \rightarrow \text{Release}$

Additional info: These notes synthesize and expand on the provided study prep slides for Ch. 19 - Viruses, following the structure and content of a standard General Biology curriculum.