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 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.

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

Example: Enveloped virus with glycoproteins, genome, capsid, and coat.

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. 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 viral 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 of viral replication within a host cell, showing attachment, entry, synthesis, assembly, and release.

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 by the lytic cycle.

• Host DNA is degraded, and new viruses are synthesized and released.

• Bacteria may have restriction enzymes to degrade viral DNA.

Lysogenic Cycle

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

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

• Prophage: Viral DNA integrated into the bacterial chromosome.

Example: Diagram 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 polymerases.

Retroviruses

Retroviruses contain RNA genomes and use reverse transcriptase to transcribe their genes into the host's DNA.

• 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 the 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, including fungi, plants, bacteria, and animals.

Example: Diagram of viral genome replication using host and viral enzymes.

Positive and Negative Sense RNA Viruses

RNA viruses are classified based on 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 the genome into dsDNA, which integrates into the host genome.

Example: Classification of viruses by genome type and replication strategy.

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

Example: Diagram showing prion conversion and accumulation.

Key Terms and Concepts

• Capsid: Protein shell of a virus.

• Capsomere: Subunit of the capsid.

• Bacteriophage: Virus that infects bacteria.

• Viral envelope: Lipid membrane surrounding some viruses.

• Host range: Spectrum of host cells a virus can infect.

• Lytic cycle: Viral replication resulting in host cell death.

• Lysogenic cycle: Viral genome integration without host cell death.

• Reverse transcriptase: Enzyme for RNA to DNA transcription.

• Viroid: Small, circular RNA pathogen.

• Prion: Infectious protein causing neurodegenerative diseases.

Important Equations and Processes

• Reverse Transcription:

• Viral Replication (Generalized):

Additional info: These notes provide a comprehensive overview of viral structure, replication, classification, and other infectious agents relevant to introductory biology and biochemistry, but are not directly focused on General Chemistry topics such as atomic structure, chemical bonding, stoichiometry, or thermodynamics.