Viruses: Structure and Classification

General Properties of Viruses

Viruses are microscopic infectious agents that require host cells for replication. 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

• Consists of a protein capsid enclosing DNA or RNA.

• Specialized structures for injecting genetic material into host cells.

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.

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

Example: Enveloped Virus

• Contains glycoproteins, genome, capsid, and coat.

Viral Infection and Replication

Entry and Infection Mechanisms

Viral infection begins when the virus binds to the host cell and its genome enters the cell. Entry mechanisms vary 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 Genome Injection

• Viral RNA or DNA is injected into the bacterial genome.

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: Viral Replication Cycle

• Attachment, entry, synthesis, assembly, and release of new viruses.

Viral Life Cycles: Lytic and Lysogenic

Lytic Cycle

The lytic cycle involves phage replication 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 both lytic and lysogenic cycles.

Example: Lytic vs. Lysogenic Cycle

• Lytic cycle results in cell lysis and release of new phages.

• Lysogenic cycle involves integration and replication with host 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 polymerase.

Retroviruses and Reverse Transcriptase

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 integrates into the host genome.

Example: HIV Replication Cycle

• Entry, reverse transcription, integration, transcription, translation, assembly, and release.

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.

Example: DNA and RNA Virus Replication

• DNA viruses use host DNA polymerase; RNA viruses use viral RNA polymerase.

Positive and Negative Sense RNA Viruses

RNA viruses are classified by the sense of their RNA:

• Positive sense RNA: Genome contains sequences needed to produce viral proteins; can be immediately translated.

• Negative sense RNA: Genome contains complementary sequences; must be transcribed by viral RNA polymerase.

Retrovirus Replication

Retroviruses use reverse transcriptase to convert RNA into DNA, which integrates into the host genome.

Example: Baltimore Classification of Viruses

Other Infectious Agents: Viroids and Prions

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.

• Prion diseases include Creutzfeldt-Jakob disease and mad cow disease.

Example: Prion Propagation

• Conversion of normal PrPC to abnormal PrPSc leads to accumulation and disease.

Key Equations and Concepts

• Viral Replication Equation:

• Reverse Transcription:

• Baltimore Classification:

Additional info: These notes expand on the original content by providing definitions, examples, and academic context for each topic, including the Baltimore classification table and key equations.