Viruses: Structure and Classification

Basic Structure of Viruses

Viruses are microscopic infectious agents that require a host cell to replicate. They are significantly smaller than cells and consist of genetic material encased in a protein shell.

• Capsid: The protein coat covering the viral genome, which can have various shapes and forms.

• Capsomere: A subunit of the capsid.

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

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

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

Viral Envelope and Host Range

Some viruses possess an additional lipid membrane called the viral envelope, derived from the host cell membrane. The host range refers to the spectrum of host cells a virus can infect, determined by specific interactions between viral surface proteins and host cell receptors.

• Glycoproteins: Surface proteins on the viral envelope that mediate host cell recognition and entry.

Example: Enveloped virus with glycoproteins, capsid, and genome.

Viral Infection and Replication

Entry and Infection Mechanisms

Viral infection begins when the virus attaches to the host cell and introduces its genetic material.

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

• Others enter via endocytosis or membrane fusion.

Example: Bacteriophage injecting DNA into a bacterial cell.

Viral Replication Cycle

Once inside, the virus hijacks the host's machinery to replicate its genome and produce viral proteins. The host provides nucleotides, enzymes, ribosomes, tRNA, amino acids, and ATP.

• Viral nucleic acids and capsid proteins are synthesized and assembled into new viruses.

Example: Diagram showing viral RNA replication and protein synthesis in a host cell.

Lytic and Lysogenic Cycles

Lytic Cycle

The lytic cycle results in the destruction of the host cell as new viruses are produced and released.

• Virulent phage: Replicates only via the lytic cycle.

• Host DNA is degraded, and viral components are synthesized and assembled.

Lysogenic Cycle

The lysogenic cycle allows viral DNA to integrate into the host genome, replicating along with it without killing the host immediately.

• Temperate phage: Can switch between lytic and lysogenic cycles.

• Prophage: Viral DNA integrated into the bacterial chromosome.

Example: Diagram comparing lytic and lysogenic cycles in bacteriophages.

Animal Viruses and Retroviruses

Animal Virus Replication

Animal viruses often have viral envelopes and RNA genomes. Entry into host cells involves recognition of cell surface proteins.

• Viral RNA can serve as a template for synthesis and replication.

Retroviruses

Retroviruses are RNA viruses that use reverse transcriptase to synthesize DNA from their RNA genome, which then integrates into the host genome.

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

Example: HIV replication cycle, showing reverse transcription and integration into host DNA.

Viral Genomes and Replication Strategies

DNA and RNA Viruses

• Double-stranded DNA viruses: Enter the nucleus for replication, often during S phase.

• Double-stranded RNA viruses: Replicate in the cytosol, using viral enzymes.

Example: Diagram of viral genome replication using host and viral enzymes.

Positive and Negative Sense RNA Viruses

• Positive sense RNA: Genome can be directly translated into proteins.

• Negative sense RNA: Genome is complementary to mRNA; requires viral RNA polymerase for transcription.

Retroviruses: Use reverse transcriptase to convert RNA into DNA for integration into the host genome.

Other Infectious Agents

Viroids

Viroids are the smallest known infectious pathogens, consisting of short, circular, single-stranded RNA. They primarily infect plants and disrupt growth.

• Viroids do not encode proteins but replicate using host enzymes.

Example: Structure of a viroid RNA molecule.

Prions

Prions are infectious proteins that cause neurodegenerative diseases in animals. They propagate by inducing misfolding in normal proteins.

• Prions can exist in multiple conformations, some of which are transmissible.

Example: Diagram showing conversion of normal PrPC protein to the infectious PrPSc form.

Summary Table: Types of Infectious Agents

Key Formulas and Concepts

• Central Dogma (for viruses):

• Viral Replication Equation (generalized):

Additional info: This content is more relevant to introductory biology or biochemistry than general chemistry, but understanding viruses and infectious agents is sometimes included in interdisciplinary science courses.