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 with 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 spectrum of host cells a virus can infect, determined by specific interactions between viral surface proteins and host cell receptors.

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

• Host range: Determined by the presence of compatible receptors on host cells.

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

Viral Infection and Replication

Entry and Genome Delivery

Viral infection begins when the virus binds to the host cell and delivers its genome inside. The method of entry varies among viruses:

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

• Other viruses enter by endocytosis or by fusion with the host 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 for viral replication.

• Nucleic acids, capsomeres, and other viral components are synthesized and assembled into new viruses.

Example: Diagram showing viral RNA replication and protein synthesis inside a host cell.

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 the destruction (lysis) of the host cell.

• Virulent phage: A phage that replicates only via the lytic cycle.

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

• Lysogenic cycle: Viral genome integrates into the host genome and replicates along with it 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 of bacteriophage infection.

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, and viral RNA may serve as a template for synthesis and replication.

• Viral envelopes facilitate entry and exit from host cells.

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 → DNA transcription.

• Integrated viral DNA is called a provirus.

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

Types of Viral Genomes and Replication Strategies

DNA and RNA Viruses

Viruses are classified by the type of nucleic acid they contain and their replication strategy.

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

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

Example: Diagram of DNA-dependent RNA polymerase transcribing viral DNA.

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; must be transcribed by viral RNA polymerase before translation.

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

Example: Classification 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. 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 brain diseases in animals. They can induce normal proteins to misfold, leading to disease. Some prion forms are transmissible to other proteins.

Example: Diagram showing conversion of normal PrPC to pathogenic PrPSc and accumulation in neural tissue.

Summary Table: Types of Infectious Agents

Key Equations and Concepts

• Central Dogma (for viruses):

• Reverse Transcription (retroviruses):

• Lytic vs. Lysogenic Cycle:

Additional info: This content is more relevant to introductory biology or biochemistry than general chemistry, but may be included in some chemistry courses covering biological molecules or biochemistry basics.