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 enclosed in a protein capsid.

Viral Envelope and Host Range

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

• Viral envelope: Contains surface proteins that attach to specific receptors on the host cell.

• Viruses identify host cells via surface protein-receptor interactions.

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 and its genome enters the cell.

• Bacteriophages inject their genome into the host.

• Some viruses are absorbed by endocytosis.

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

Example: Bacteriophage injecting viral DNA 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 showing viral RNA replication and protein synthesis 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 the lytic and lysogenic cycles of bacteriophage replication.

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 that are reverse transcribed into DNA and integrated into the host genome.

• 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 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.

• 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.

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 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 RNA into DNA, which is then integrated into the host genome.

Example: Classification diagram 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.

• Mostly infect plants and disrupt growth.

• Do not encode proteins; 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.

• Prions can fold in multiple ways, some of which are transmissible to other proteins.

• Cause diseases such as Creutzfeldt-Jakob disease and mad cow disease.

Example: Diagram showing conversion of normal PrP protein to infectious PrPSc form and its accumulation.

Summary Table: Types of Infectious Agents

Key Equations and Concepts

• Central Dogma of Molecular Biology:

• Reverse Transcription (Retroviruses):

• Viral Replication Cycle: