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, which is derived from the host cell membrane and contains viral glycoproteins.

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

• Host range: The spectrum of host cells that a virus can infect, determined by specific interactions between viral surface proteins and host cell receptors.

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

Viral Infection and Replication

Entry and Infection Mechanisms

Viral infection begins when the virus binds to the host cell, allowing its genome to enter the cell. The method of entry varies 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 using 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 within a host cell.

Viral Life Cycles

Lytic Cycle

The lytic cycle involves phage replication that results in the destruction 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 cell.

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

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

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

Animal Viruses and Retroviruses

Animal Virus Structure and 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 and Reverse Transcriptase

Retroviruses contain RNA genomes and use reverse transcriptase to transcribe their RNA into DNA, which integrates into the host genome.

• Reverse transcriptase: Enzyme that catalyzes RNA to DNA transcription.

• Integrated viral DNA is called a provirus.

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

Viral Genomes and Classification

DNA and RNA Viruses

Viruses are classified based on their genetic material and replication strategies.

• Double-stranded DNA viruses: Enter the nucleus for replication, often during S phase.

• Double-stranded RNA viruses: Enter the cytosol and use viral enzymes for genome replication.

Example: Diagram of DNA and RNA virus replication in host cells.

Positive and Negative Sense RNA Viruses

RNA viruses are further classified by the sense of their RNA genome.

• Positive sense RNA: Genome can be directly translated into proteins.

• Negative sense RNA: Genome is complementary to mRNA and must be transcribed by viral RNA polymerase.

• Retrovirus (+ssRNA): Uses reverse transcriptase to produce dsDNA from RNA.

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 molecules. They primarily infect plants and disrupt growth.

• Viroids do not encode proteins but replicate using host enzymes.

Example: Structure of a viroid RNA molecule.

Prions

Prions are infectious proteins that cause neurodegenerative diseases in animals by inducing abnormal folding of normal proteins.

• Prions can propagate by converting normal proteins into the prion form.

• Some prion diseases are transmissible between individuals.

Example: Diagram showing prion propagation and accumulation in brain tissue.

Summary Table: Viral Genome Types and Replication Strategies

Key Equations and Concepts

• Central Dogma (for viruses):

• Reverse Transcription (Retroviruses):

• Viral Replication Rate: (Generalized)

Additional info: These notes provide a comprehensive overview of viral structure, replication, and classification, as well as other infectious agents such as viroids and prions. While the content is more closely related to biology and biochemistry, understanding viral structure and replication is relevant to General Chemistry students, especially in the context of molecular structure, chemical interactions, and the role of enzymes and nucleic acids.