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

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 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 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 interactions between viral surface proteins and host cell receptors.

• Viral envelope: Accessory structure common in animal viruses, often derived from host cell membranes.

• Host range is determined by the ability of viral proteins to bind specific receptors on host cells.

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

Viral Infection and Replication

Entry and Genome Release

Viral infection begins when the virus binds to the host cell, allowing the viral genome to enter the cell. The method of entry varies 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 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 spontaneously assemble into new viruses.

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

Phage Replication Cycles

Lytic Cycle

The lytic cycle is a viral replication process 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 viral genome to replicate without killing the host.

• 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 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 have RNA genomes that are reverse transcribed into DNA, which integrates into the host genome.

• 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. 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.

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)

Retroviral reverse transcriptase enters the cell with the genome and transcribes the dsDNA version of the genome.

Example: Classification of viral genomes (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.

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.

• Diseases caused by prions include Creutzfeldt-Jakob disease and mad cow disease.

Summary Table: Types of Infectious Agents

Agent	Genetic Material	Host Range	Replication Method
Virus	DNA or RNA	Bacteria, plants, animals	Uses host cell machinery
Viroid	RNA (short, circular)	Plants	Uses host enzymes
Prion	None (protein only)	Animals	Induces misfolding of host proteins

Additional info: The notes expand on the basic structure and replication of viruses, including bacteriophages, animal viruses, and retroviruses, and introduce viroids and prions as other infectious agents relevant to General Biology (Ch. 19).