Viruses: Structure and Classification

General Properties of Viruses

Viruses are microscopic infectious agents that require a host cell to replicate. They are significantly smaller than cells and act as vessels for genetic material.

• Capsid: The protein coat covering the viral genome, which may take many different forms.

• Capsomere: A subunit of the capsid.

• Viruses may contain double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA.

• Bacteriophages: Viruses that infect bacteria and contain complex capsids.

Example: Bacteriophage structure showing DNA and protein components.

Viral Envelope and Host Range

Some viruses possess an accessory structure called a viral envelope, which is derived from the host cell membrane and contains viral glycoproteins.

• Viral envelope: Surrounds the capsid in many animal viruses, aiding in host cell entry.

• Host range: The collection of hosts that a virus can enter and infect, determined by specific surface proteins that attach to receptors on host cells.

Example: Animal virus with glycoproteins, genome, capsid, and coat.

Viral Infection and Replication

Entry and Infection Mechanisms

Viral infection begins when the virus binds to the host cell and its genome enters the cell.

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

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

Example: Bacteriophage injecting viral DNA into a bacterial genome.

Viral Replication

Once inside, the virus hijacks the host's replicative machinery to produce viral components from its own genes.

• The host provides nucleotides, enzymes, ribosomes, tRNA, amino acids, and ATP.

• Nucleic acids and capsomeres are produced and assemble into new viruses.

Example: Diagram showing viral replication and assembly in a host cell.

Viral Life Cycles

Lytic Cycle

The lytic cycle is a viral replication process that results in the destruction of the host cell.

• The phage inserts its DNA, degrades host DNA, and synthesizes viral components.

• Virulent phage: Replicates only by the lytic cycle.

• Bacteria may have restriction enzymes that degrade viral DNA.

Lysogenic Cycle

The lysogenic cycle allows the viral genome to replicate without killing the host.

• The phage DNA integrates into the host chromosome as a prophage.

• Temperate phage: Capable of replicating through both lytic and lysogenic cycles.

Example: Diagram comparing lytic and lysogenic cycles.

Animal Viruses and Retroviruses

Animal Virus Structure and Replication

Animal viruses often have viral envelopes and RNA genomes. Replication involves entry via cell surface protein-receptor recognition.

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

Retroviruses and Reverse Transcriptase

Retroviruses contain RNA genomes and use reverse transcriptase to transcribe their genes into the host's DNA.

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

• Viral DNA integrates into the host genome.

Example: HIV replication cycle showing reverse transcription and integration.

Viral Genomes: DNA and RNA Viruses

Double-Stranded DNA Viruses

These viruses enter the nucleus to replicate, often during S phase of the cell cycle.

• Infect a wide array of organisms except plant viruses, which are usually single-stranded.

Double-Stranded RNA Viruses

These viruses enter the cytosol and use viral enzymes to replicate their genome.

• Infect a wide variety of organisms, including fungi, plants, vertebrates, bacteria, and insects.

Positive and Negative Sense RNA Viruses

• Positive sense RNA virus: Genome contains the same sequences needed to produce viral proteins; genome enters cell and is immediately translated.

• Negative sense RNA virus: Genome contains complementary sequences; viral RNA polymerase must transcribe RNA to produce viral proteins.

• Retrovirus (+ssRNA): Reverse transcriptase transcribes RNA genome into dsDNA, which integrates into the host genome.

Example: Diagram showing viral genome replication and classification.

Other Infectious Agents

Viroids

Viroids are the smallest known pathogens, consisting of short, circular, single-stranded RNA.

• Mostly infect plants and disrupt growth.

• Do not encode proteins; replicate in the host using host enzymes.

Example: Structure of a viroid RNA molecule.

Prions

Prions are infectious self-propagating proteins that cause brain diseases in animals.

• Prions can fold in multiple ways, some of which are transmissible to other proteins.

• Cause diseases such as Creutzfeldt-Jakob disease and mad cow disease.

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

Summary Table: Virus Genome Types and Replication Strategies

Key Equations and Concepts

• Central Dogma of Molecular Biology (for viruses):

• Reverse Transcription (Retroviruses):

• Positive Sense RNA Virus Translation:

• Negative Sense RNA Virus Transcription:

Additional info: Academic context and definitions have been expanded for clarity and completeness. Diagrams referenced in examples are based on standard textbook illustrations of viral structure and replication.