Viruses: Structure and Classification

General Characteristics 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 often contain 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.

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

Example: Animal virus with glycoproteins, capsid, genome, and envelope.

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.

• Some viruses, like bacteriophages, inject their genome into the host.

• Other viruses are absorbed by endocytosis or 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.

• Nucleic acids and capsomeres are produced and assemble into new viruses.

Example: Diagram of viral replication inside a host cell, showing synthesis and assembly of viral components.

Viral Life Cycles

Lytic and Lysogenic Cycles

Viruses, especially bacteriophages, can follow two main replication cycles:

• Lytic cycle: Viral replication that culminates in the death of the host cell.

  • The phage inserts its DNA, degrades host DNA, and synthesizes viral components.

  • Virulent phage: Replicates only by the lytic cycle.

  • Many bacteria have restriction enzymes that degrade viral DNA as a defense.

• Lysogenic cycle: Viral genome replicates without killing the host.

  • The phage DNA integrates into the host chromosome as a prophage.

  • Temperate phage: Capable of 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 polymerase.

Retroviruses

Retroviruses are RNA viruses that reverse transcribe their genes into the host's DNA chromosome.

• Reverse transcriptase: The enzyme that catalyzes RNA to DNA transcription.

• The viral DNA is integrated into the host genome.

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

Viral Genomes and Replication Strategies

DNA and RNA Viruses

• Double-stranded DNA viruses: Enter the nucleus to replicate, often during S phase of the cell cycle.

• Double-stranded RNA viruses: Enter the cytosol and use viral enzymes to replicate their genome.

Viruses can infect a wide variety of organisms, including fungi, plants, vertebrates, bacteria, and insects.

Positive and Negative Sense RNA Viruses

• Positive sense RNA virus: Genome contains the same sequence needed to produce viral proteins; can be directly translated.

• Negative sense RNA virus: Genome contains the complementary sequence; must be transcribed by viral RNA polymerase before translation.

• Retrovirus (+ssRNA): Reverse transcriptase transcribes the RNA genome into dsDNA, which integrates into the host genome.

Example: Baltimore classification of viruses based on genome type (dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA, etc.).

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.

Example: Structure of a viroid RNA molecule.

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.

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

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

Summary Table: Virus Types and Replication Strategies

Additional info: This content is more relevant to introductory biology or biochemistry than general chemistry, but understanding viruses and infectious agents is sometimes included in interdisciplinary science courses.