Viruses and Other Infectious Agents: Structure, Replication, and Classification

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 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 showing DNA enclosed in a protein capsid.

Viral Envelope and Host Range

Some 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 interactions between viral surface proteins and host cell receptors.

• Viral envelope: Accessory structure common in animal viruses.

• Viruses identify host cells via surface proteins that attach to specific receptors.

Example: Animal virus with glycoproteins embedded in the envelope, surrounding the capsid and genome.

Viral Infection and Replication

Entry and Genome Injection

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

• 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 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 for viral replication.

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

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

Viral Life Cycles

Lytic Cycle

The lytic cycle involves replication of viral DNA, synthesis of viral components, and destruction of the host cell.

• Virulent phage: Replicates only via the lytic cycle.

• Bacterial defenses include restriction enzymes that degrade viral DNA.

Lysogenic Cycle

The lysogenic cycle allows the viral genome to integrate into the host chromosome without killing the host.

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

• Prophage: Viral DNA integrated into the bacterial chromosome.

Example: Flowchart comparing the lytic and lysogenic cycles.

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

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 is integrated 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 the S phase of the cell cycle. They infect a wide array of organisms except plants.

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

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

Other Infectious Agents

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: 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.

• They cause diseases by inducing abnormal folding of normal proteins.

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

Summary Table: Types of Infectious Agents

Agent	Genetic Material	Structure	Host Range
Virus	DNA or RNA	Capsid, sometimes envelope	Bacteria, plants, animals, fungi
Viroid	RNA (ss, circular)	No capsid or envelope	Plants
Prion	None	Protein only	Animals

Additional info: These topics are foundational for understanding the molecular biology of infectious agents and their relevance to biochemistry and cell biology. While not strictly General Chemistry, they are often included in introductory college science courses for context.