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 additional structure called the viral envelope, which is derived from the host cell membrane and contains viral glycoproteins.

• Viral envelope: Accessory structure common in animal viruses, often derived from the membrane of host cells.

• Host range: The collection of hosts that the virus can enter and infect. Viruses identify host cells via surface proteins that attach to specific receptors on the host.

Example: Animal virus with glycoprotein spikes, capsid, genome, and coat.

Viral Infection and Replication

Entry and Genome Integration

Viral infection begins when the virus binds to the host cell, allowing the viral genome to enter the cell.

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

• Some viruses are absorbed into the host by endocytosis.

• Some viruses fuse their membranes with the host's membrane.

Example: Bacteriophage injecting viral genome into a bacterial cell.

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 of viral replication inside a host cell, showing synthesis and assembly of viral components.

Viral Life Cycles

Lytic and Lysogenic Cycles

Viruses, especially bacteriophages, can replicate via two main cycles: lytic and lysogenic.

• Lytic cycle: Phage replication that culminates in the death of the host. The phage inserts its DNA, degrades host DNA, and synthesizes viral components.

• Virulent phage: A phage that replicates only by the lytic cycle.

• Many bacteria have restriction enzymes that degrade viral DNA as a defense.

• Lysogenic cycle: Replicates the viral genome without killing the host. The phage DNA integrates into the bacterial chromosome as a prophage.

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

Example: Flowchart comparing the lytic and lysogenic cycles, showing integration and release of viral DNA.

Animal Viruses and Retroviruses

Animal Virus Replication

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

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

Retroviruses

Retroviruses are RNA viruses that reverse transcribe their genes into the host's DNA chromosome.

• Reverse transcriptase: The enzyme that catalyzes RNA to DNA transcription.

• Provirus: The viral DNA integrated into the host genome.

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

Viral Genomes and Replication Strategies

DNA and RNA Viruses

Viruses can have double-stranded DNA, double-stranded RNA, or single-stranded RNA genomes, each with distinct replication strategies.

• Double-stranded DNA virus: Enters nucleus to replicate, often during S phase of the cell cycle.

• Double-stranded RNA virus: Enters cytosol and uses viral enzymes to replicate its genome.

Example: Diagram of DNA and RNA virus replication, showing coding and template strands.

Positive and Negative Sense RNA Viruses

Single-stranded RNA viruses are classified as positive or negative sense, depending on the sequence orientation.

• 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 proteins.

• Retrovirus (+ssRNA): Reverse transcriptase enters cell with the genome and transcribes dsDNA version of the genome.

Example: Classification chart of viral genome types (dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA, etc.).

Other Infectious Agents

Viroids

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

• Viroids do not encode proteins but 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. They can fold in multiple ways, some of which are transmissible to other proteins.

• Prions do not contain nucleic acids and propagate by inducing misfolding in normal proteins.

Example: Diagram showing conversion of normal PrPC to pathogenic PrPSc and accumulation in brain tissue.

Summary Table: Types of Infectious Agents

Key Equations and Concepts

• Central Dogma (for viruses):

• Reverse Transcription (Retroviruses):

• Viral Replication Cycle:

Additional info: These notes are relevant for introductory biology and biochemistry courses, but not directly for General Chemistry. However, the molecular structure and replication mechanisms of viruses may be discussed in biochemistry or molecular biology modules within a chemistry curriculum.