Viruses: Structure and Function

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 contain complex capsids.

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 surface proteins.

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

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

Example Structure of a Virus

• Components: Glycoprotein, Genome, Capsid, Coat

Viral Infection and Replication

Entry and Infection Mechanisms

Viral infection begins when the virus binds to the host cell and the viral genome enters the cell. Entry mechanisms vary among viruses.

• Bacteriophages inject their genome into the host.

• Some viruses are absorbed into the host by endocytosis.

• Some viruses fuse their membranes with the host's membrane.

Example: Bacteriophage Infection

• 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.

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

Example: Viral Replication Cycle

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

Lytic and Lysogenic Cycles

Lytic Cycle

The lytic cycle involves replication of viral DNA that culminates in the death of the host cell.

• The phage inserts its DNA, host DNA is degraded, and viral components are synthesized.

• 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.

• The phage DNA is integrated into the bacterial chromosome as a prophage.

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

Example: Lytic vs. Lysogenic Cycle

• Lytic: Immediate replication and cell lysis.

• Lysogenic: Integration and dormancy within host genome.

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 have RNA genomes that are reverse transcribed into DNA and integrated into the host's chromosome.

• Reverse transcriptase: Enzyme that catalyzes RNA to DNA transcription.

• Viral DNA is integrated into the host genome.

Example: HIV Replication Cycle

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

Viral Genomes and Replication Strategies

DNA and RNA Viruses

Viruses are classified by their genome type and replication strategy.

• Double-stranded DNA viruses: Enter nucleus, replicate during S phase.

• Double-stranded RNA viruses: Enter cytosol, use viral enzymes for synthesis.

Positive and Negative Sense RNA Viruses

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

• Negative sense RNA virus: Genome must be transcribed into complementary RNA before translation.

• Retrovirus (+ssRNA): Reverse transcriptase transcribes RNA into DNA.

Example: Baltimore Classification of Viruses

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

• Prions do not contain nucleic acids.

• Diseases include mad cow disease and Creutzfeldt-Jakob disease.

Example: Prion Propagation

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

Key Equations and Concepts

• Viral Replication Equation:

• Reverse Transcription: (catalyzed by reverse transcriptase)

Additional info: These notes cover the diversity, structure, and replication of viruses, as well as other infectious agents such as viroids and prions, relevant to General Biology topics on prokaryotes, viruses, and infectious disease.