Viruses: Structure and Classification

Introduction to Viruses

Viruses are microscopic infectious agents that are significantly smaller than cells and require a host to replicate. They act as vessels for genetic material and can infect a wide range of organisms.

• Capsid: The protein coat covering the viral genome, which may take various 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 often have complex capsids.

Example:

• Bacteriophage structure: DNA enclosed in a protein capsid, with tail fibers for host attachment.

• Animal virus structure: Genome surrounded by capsid, coat, and glycoproteins.

Viral Envelope and Host Range

Some animal viruses possess an additional structure called the viral envelope, derived from the host cell membrane. The host range refers to the collection of hosts that a virus can infect, determined by specific surface proteins.

• Viral envelope: Accessory structure common in animal viruses.

• Host range is determined by viral surface proteins that attach to specific host cell receptors.

Example:

• Enveloped virus with glycoproteins, genome, capsid, and coat.

Viral Infection and Replication

Viral Entry and Genome Injection

Viral infection begins when the virus binds to the host cell and its genome enters the cell. Entry mechanisms vary among viruses.

• Bacteriophages inject their genome into the host.

• Some viruses are absorbed by endocytosis.

• Others 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 using 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 attachment, entry, replication, protein synthesis, and release.

Viral Life Cycles: Lytic and Lysogenic

Lytic Cycle

The lytic cycle involves phage replication that results in the death of the host cell.

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

• Virulent phage: Replicates only by the lytic cycle.

• Bacteria may have restriction enzymes to degrade viral DNA.

Lysogenic Cycle

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

• Phage DNA integrates into the bacterial chromosome as a prophage.

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

Example:

• Diagram showing the lytic and lysogenic cycles, with integration and excision of viral DNA.

Animal Viruses and Retroviruses

Animal Virus 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 polymerases.

Retroviruses

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 into host DNA.

Viral Genomes: DNA and RNA Viruses

Double-Stranded DNA Viruses

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

• Infect a wide array of organisms except plant viruses.

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.

Example:

• Diagram showing RNA polymerase transcribing viral RNA.

Positive and Negative Sense RNA Viruses

RNA viruses are classified by the sense of their RNA genome.

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

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

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

Example:

• Diagram showing classification of viral genomes (dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA, etc.).

Other Infectious Agents: Viroids and Prions

Viroids

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

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

Example:

• Diagram of a viroid RNA structure.

Prions

Prions are infectious self-propagating proteins that cause brain diseases in animals. They can fold in multiple ways, some of which are transmissible to other proteins.

• Prions do not contain nucleic acids and propagate by inducing abnormal folding of normal proteins.

Example:

• Diagram showing conversion of normal PrPC to abnormal PrPSc and accumulation in the brain.

Summary Table: Virus Classification

Key Equations and Concepts

• Viral Replication Equation:

• Reverse Transcription: (catalyzed by reverse transcriptase)

Additional info: These notes expand on the original content by providing definitions, examples, and a summary table for virus classification, ensuring completeness and clarity for General Chemistry students.