Comprehensive Study Notes on Viral Structure, Life Cycle, and Pathogenesis

General Characteristics & General Morphology

Overview of Viral Structure

Viruses are unique infectious agents that differ fundamentally from cellular life forms. They possess distinct structural and functional features:

• Not cells (no cell membrane): Viruses lack cellular organization and do not possess a cell membrane.

• Chromosome, capsid, envelope: The viral genome is enclosed within a protein shell called the capsid. Some viruses also have a lipid bilayer envelope with virus-encoded proteins embedded.

• Limited metabolic machinery: Viruses have very limited metabolic capabilities and rely on host cell machinery for energy, amino acids, nucleotides, etc.

• Genome composition: The mature virus particle (virion) contains only one type of nucleic acid (DNA or RNA), always packaged with the capsid. There is no division; viruses assemble from components.

Example:

Influenza virus has an RNA genome, a capsid, and a lipid envelope derived from the host cell membrane, with viral glycoproteins embedded.

General Physiology

Viral Particle Machinery

Viruses hijack the host cell's metabolic machinery to synthesize viral components. The process involves:

• Replication enzymes and substrates: DNA or RNA nucleotides are used for genome replication.

• Transcription enzymes and substrates: RNA nucleotides are used for mRNA synthesis.

• Translation apparatus: Host ribosomes, tRNAs, and amino acids synthesize viral proteins.

• Energy: ATP is required for biosynthetic processes.

Assembly

Viral components (genome and capsid proteins) are synthesized and then assembled into new virions. This is distinct from cell division, as viruses do not divide but assemble from parts.

Viral Life Cycle

Attachment and Entry

Viruses attach to specific host cell receptors, a process that determines host range and tissue specificity. Enveloped viruses often fuse with the cell membrane, while non-enveloped viruses may enter by endocytosis.

• Attachment: Specific interaction with host cell surface proteins.

• Penetration: Entry via fusion (enveloped viruses) or endocytosis (non-enveloped viruses).

• Uncoating: Release of the viral genome into the host cell cytoplasm.

Replication and Synthesis

The viral genome is replicated and transcribed using host or viral enzymes, depending on the type of virus.

• DNA viruses: Use host or viral DNA polymerases for replication.

• RNA viruses: Use viral RNA-dependent RNA polymerases or reverse transcriptase (retroviruses).

Assembly and Release

New viral particles are assembled from synthesized components. Enveloped viruses acquire their envelope from the host cell membrane during release.

• Assembly: Capsid proteins and genome combine to form mature virions.

• Release: Virions exit the cell by budding (enveloped viruses) or cell lysis (non-enveloped viruses).

Viral Genome Types and Replication Strategies

dsDNA Viruses

Double-stranded DNA viruses replicate their genome and transcribe mRNA using host cell machinery.

• Host RNA polymerase transcribes viral genes.

• Host ribosomes translate mRNA into viral proteins.

• Viral DNA is replicated.

• Capsid proteins are synthesized and assembled.

ssRNA Viruses

Single-stranded RNA viruses have diverse replication strategies:

• Positive-sense (+) RNA: Acts directly as mRNA; host ribosomes translate viral proteins. Viral RNA-dependent RNA polymerase synthesizes complementary (-) RNA for genome replication.

• Negative-sense (-) RNA: Must be transcribed into complementary (+) RNA by viral RNA-dependent RNA polymerase before translation.

• Retroviruses: Use reverse transcriptase to synthesize complementary DNA (cDNA) from RNA, which integrates into the host genome.

Example Table: Replication Strategies

Virus Type	Genome	Key Enzyme	Replication Strategy
dsDNA	Double-stranded DNA	DNA polymerase	Transcription by host RNA polymerase, replication by DNA polymerase
+ssRNA	Single-stranded RNA (+)	RNA-dependent RNA polymerase	Direct translation, synthesis of complementary (-) RNA
-ssRNA	Single-stranded RNA (-)	RNA-dependent RNA polymerase	Transcription to (+) RNA, then translation
Retrovirus	Single-stranded RNA	Reverse transcriptase	Reverse transcription to DNA, integration into host genome

Viruses and Cancer

Oncogenic Viruses

Certain viruses can induce cancer by integrating their genome into host cells and disrupting normal cell cycle regulation.

• HPV (Human Papillomavirus): Causes cervical cancer by expressing oncogenes that interfere with cell cycle control.

• Retroviruses: Some retroviruses carry oncogenes or activate host proto-oncogenes, leading to uncontrolled cell division.

Example:

HPV infection is a major risk factor for cervical cancer. Vaccination against HPV can prevent infection and reduce cancer incidence.

Prions

Nature and Pathogenesis

Prions are infectious proteins that cause neurodegenerative diseases in animals and humans. They lack nucleic acids and propagate by inducing misfolding of normal host proteins.

• Diseases: Scrapie (sheep), Bovine Spongiform Encephalopathy (BSE, "mad cow disease"), Creutzfeldt-Jakob disease (humans).

• Mechanism: Prions convert normal proteins into abnormal, disease-causing forms.

Example:

BSE outbreaks in cattle have led to human cases of variant Creutzfeldt-Jakob disease through consumption of contaminated beef.

Viroids

Characteristics

Viroids are small, circular RNA molecules that infect plants. They do not encode proteins and rely entirely on host machinery for replication.

• Genome: 300-400 nucleotides, no protein-coding capacity.

• Transmission: Spread by insects or mechanical means, causing plant diseases.

Example:

Potato spindle tuber viroid causes stunted growth and yield loss in potatoes.

Coat Protein Mediated Resistance

Biotechnological Application

Genetic engineering can confer resistance to plant viruses by expressing viral coat proteins in transgenic plants. This disrupts viral assembly and prevents infection.

• Example: Grapevine fan leaf virus resistance in transgenic grape plants expressing the coat protein gene.

• Mechanism: High levels of coat protein in the plant cell prevent proper assembly of viral particles, stopping infection.

Additional info:

This strategy has been used to protect crops from devastating viral diseases and is a key concept in plant virology and biotechnology.

Key Equations and Concepts

• Viral replication rate: $\text{Rate} = k[\text{Viral genome}][\text{Host machinery}]$

• Reverse transcription (retroviruses): $\text{RNA} \xrightarrow{\text{reverse transcriptase}} \text{cDNA}$