Viruses: Structure and Classification

General Properties of Viruses

Viruses are microscopic infectious agents that require host cells 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 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: DNA enclosed in a protein capsid with tail fibers for host attachment.

• Animal virus structure: Spherical capsid surrounded by a viral envelope and glycoproteins.

Viral Envelope and Host Range

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

• Viral envelope: Contains host-derived lipids and viral glycoproteins.

• Host specificity is determined by viral surface proteins that bind to receptors on host cells.

Example:

• Influenza virus with envelope and glycoproteins for host cell recognition.

Viral Infection and Replication

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 cell.

• Some viruses are absorbed by endocytosis.

• Others fuse their membranes with the host's membrane.

Example:

• Bacteriophage injecting DNA into a bacterial cell.

Viral Replication

Once inside, the virus hijacks the host's replicative machinery to produce viral components. 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 RNA replication, protein synthesis, and assembly in a host cell.

Phage Life Cycles: Lytic and Lysogenic

Lytic Cycle

The lytic cycle involves phage replication that culminates 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 that degrade viral DNA.

Lysogenic Cycle

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

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

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

Example:

• Diagram comparing lytic and lysogenic cycles, showing 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 polymerase.

Retroviruses and Reverse Transcriptase

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

• 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, often during S phase of the cell cycle. They 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. They infect a variety of organisms, including fungi, plants, vertebrates, bacteria, and insects.

Example:

• Diagram of viral genome replication using host and viral enzymes.

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 viral RNA into dsDNA, which integrates into the host genome.

Example:

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

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 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 disrupt normal protein folding and accumulate in neural tissue.

Example:

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

Summary Table: Virus Genome Types and Replication Strategies

Additional info: This table summarizes the main types of viral genomes, their replication sites, and key enzymes involved, providing a quick reference for exam preparation.