Viruses: Structure and Classification

Basic Structure of Viruses

Viruses are microscopic infectious agents that require a host cell to replicate. They are significantly smaller than cells and consist of genetic material encased in a protein shell.

• Capsid: The protein coat covering the viral genome, which can 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 as their genetic material.

• Bacteriophages: Viruses that infect bacteria and often have complex structures.

Example: Bacteriophage structure showing DNA inside a protein capsid and tail fibers for host attachment.

Viral Envelope and Host Range

Some viruses possess an additional lipid membrane called the viral envelope, derived from the host cell membrane. The host range refers to the specific set of organisms or cell types a virus can infect, determined by interactions between viral surface proteins and host cell receptors.

• Viral envelope: Accessory structure common in animal viruses, often containing glycoproteins.

• Host range: Determined by compatibility between viral proteins and host cell receptors.

Example: Enveloped virus with 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 introduces its genetic material.

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

• Other viruses enter by endocytosis or by fusing with the host membrane.

Example: Bacteriophage injecting DNA into a bacterial cell.

Viral Replication

Once inside, the virus hijacks the host's cellular machinery to replicate its genome and produce new viral components.

• The host provides nucleotides, enzymes, ribosomes, tRNA, amino acids, and ATP.

• Viral nucleic acids and capsid proteins are synthesized and assembled into new viruses.

Example: Diagram showing viral replication, including attachment, entry, synthesis, assembly, and release.

Viral Life Cycles

Lytic and Lysogenic Cycles

Bacteriophages can follow two main life cycles: the lytic cycle and the lysogenic cycle.

• Lytic cycle: Viral replication leads to host cell lysis and death.

• Virulent phage: Replicates only via the lytic cycle.

• Host bacteria may have restriction enzymes to degrade viral DNA.

• Lysogenic cycle: Viral genome integrates into the host chromosome as a prophage and replicates with the host without killing it.

• Temperate phage: Can replicate via both lytic and lysogenic cycles.

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

Animal Viruses and Retroviruses

Animal Virus Replication

Animal viruses often have envelopes and RNA genomes. Replication involves entry via cell surface protein-receptor recognition, and viral RNA may serve as a template for synthesis and replication.

• Viral RNA acts as a template for synthesis and replication by viral RNA polymerase.

Retroviruses and Reverse Transcriptase

Retroviruses are RNA viruses that use reverse transcriptase to synthesize DNA from their RNA genome, which then 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 Replication Strategies

DNA and RNA Viruses

Viruses are classified by their nucleic acid type and replication strategy.

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

• Double-stranded RNA viruses: Replicate in the cytosol, using viral enzymes.

• Viruses infect a wide range of organisms, including fungi, plants, animals, and bacteria.

Example: Diagram of RNA polymerase transcribing viral genome.

Positive and Negative Sense RNA Viruses

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

• Negative sense RNA virus: Genome is complementary to mRNA; requires viral RNA polymerase to synthesize mRNA.

• Retrovirus (+ssRNA): Uses reverse transcriptase to make DNA from RNA, which integrates into the host genome.

Example: Classification of viral genomes (dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA, ssRNA-RT, dsDNA-RT).

Other Infectious Agents

Viroids

Viroids are the smallest known infectious pathogens, consisting of short, circular, single-stranded RNA. They primarily infect plants and disrupt growth, replicating using host enzymes without encoding proteins.

Example: Structure of a viroid RNA molecule.

Prions

Prions are infectious, self-propagating proteins that cause neurodegenerative diseases in animals. Prions can induce normal proteins to misfold, leading to disease. Some prion forms are transmissible between individuals.

Example: Diagram showing conversion of normal PrPC to misfolded PrPSc and accumulation in the brain.

Summary Table: Types of Infectious Agents

Key Equations and Terms

• Reverse Transcription:

• Transcription:

• Translation:

Additional info: This content is more relevant to introductory biology or biochemistry than general chemistry, but understanding viruses and infectious agents is important for students in life sciences and related chemistry fields.