Viral Molecular Biology: Structure, Replication, and Pathogenesis

Viral Structure and Classification

Capsid, Nucleocapsid, and Viral Envelopes

Viruses are composed of a nucleic acid genome encased in a protein shell called the capsid. The combination of the capsid and the genome is termed the nucleocapsid. Some viruses possess an additional lipid membrane called the envelope, derived from the host cell membrane, while others are naked viruses lacking this envelope.

• Capsid: Protects viral genetic material from nucleases and environmental damage.

• Envelope: Contains viral glycoproteins essential for host cell recognition and entry.

• Receptor-binding proteins: Mediate attachment to specific host cell receptors, determining host range.

Example: HIV is an enveloped virus, while poliovirus is a naked virus.

Viral Genes and Dependency on Host

Genes Required for Infectious Virus Assembly

Viruses encode genes for structural proteins (capsid, envelope proteins), enzymes (such as polymerases), and receptor-binding proteins. Genome length and composition vary widely among viruses (e.g., ssRNA, dsRNA, DNA viruses, and large "giruses"). Viruses are obligate intracellular parasites, relying on host cell machinery for replication.

• Capsid protein genes: Form the protective shell.

• Receptor-binding protein genes: Enable host cell attachment.

• Polymerase genes: Required for genome replication, especially in RNA viruses.

Viral Replication Cycle

Overview of Viral Replication

Viral replication is a multi-step process involving attachment, entry, uncoating, replication, assembly, and release. Viruses hijack host cell machinery to produce progeny virions.

Attachment

Viral surface proteins bind to specific receptors on the host cell membrane, determining host and tissue specificity (tropism).

• Specificity: Lock-and-key interaction; e.g., HIV binds CD4 on T-helper cells.

Entry (Penetration)

Viruses enter host cells via several mechanisms:

• Membrane Fusion: Enveloped viruses fuse with the host membrane, releasing the nucleocapsid into the cytoplasm.

• Endocytosis: Virus is engulfed by the cell, forming a vesicle that transports it inside.

• Genetic Injection: Bacteriophages inject their genome into the host, leaving the capsid outside.

Uncoating

After entry, the viral capsid is removed, releasing the genome into the host cell. This process is often triggered by acidic conditions in endosomes.

Replication and Biosynthesis

The viral genome directs the host cell to synthesize viral proteins and replicate the viral genome. The location of replication depends on the type of virus (DNA viruses often replicate in the nucleus; RNA viruses in the cytoplasm).

• Viral protein production: Synthesis of enzymes and structural proteins.

• Genome replication: Host machinery is used to make new copies of the viral genome.

Assembly (Maturation)

New viral genomes and proteins are assembled into new virions. This process may occur in the cytoplasm or nucleus, depending on the virus.

Release (Egress)

Newly formed viruses exit the host cell by:

• Lysis: Host cell bursts, releasing virions (common in non-enveloped viruses).

• Budding: Enveloped viruses acquire their envelope by budding from the host membrane, allowing continued cell survival for a time.

Molecular Hurdles of the Host Cell

Barriers to Viral Infection

Eukaryotic cells present several molecular barriers to viral infection:

• Receptors and polymerases: Host polymerases cannot replicate viral RNA genomes; viruses encode their own RNA-dependent RNA polymerases.

• Actin remodeling: Affects viral entry, movement, and egress.

• Ribosome compatibility: Host ribosomes translate monocistronic mRNAs; viruses use alternative splicing, segmented genomes, or polyproteins to overcome this.

• mRNA competition: Viruses evolve mechanisms to prioritize their own protein synthesis over host proteins.

Genetic Diversity in RNA Viruses

RNA viruses mutate rapidly due to the lack of proofreading by RNA polymerases, leading to high genetic diversity and adaptability.

Targets for Antiviral Therapies

Effective antiviral drugs target essential viral processes (e.g., replication enzymes) while minimizing toxicity to the host.

Mechanisms of Viral Entry and Spread in the Body

Preferred Routes of Entry

Viruses enter the body through various routes, each with unique challenges and adaptations:

• Respiratory tract: Inhalation of aerosolized droplets (e.g., influenza, rhinovirus).

• Gastrointestinal tract: Ingestion via contaminated food/water; viruses must resist acidic pH (e.g., norovirus).

• Genital tract: Entry through abrasions or infection of reproductive tract cells (e.g., HIV, HPV).

• Conjunctiva: Infection of the eye's surface (e.g., enterovirus 70, Zika virus).

• Skin: Requires breach (e.g., cuts, bites, needle punctures).

• Transplacental: Some viruses cross the placenta to infect the fetus.

Mechanisms of Viral Spread and Pathogenesis

After entry, viruses may spread locally or systemically:

• Local spread: Direct cell-to-cell transmission or release of virions to neighboring cells.

• Systemic spread: Via bloodstream (hematogenous) or nerves (neurotropic spread).

• Vertical transmission: From mother to child (transplacental, perinatal, postnatal).

Virus Exit: Shedding

Viruses are shed from the body via their entry routes or other organs, facilitating transmission to new hosts. Zoonotic viruses (transmitted from animals) are of particular concern due to their potential for emergence.

Environmental Survival of Viruses

Virus survival outside the host depends on factors such as composition, temperature, humidity, and pH. Enteric viruses are particularly stable in water environments, posing risks for waterborne outbreaks.

Patterns of Viral Disease

Types of Viral Infections

• Acute infection: Rapid onset and resolution.

• Acute infection with persistent latent phase: Virus remains dormant and can reactivate.

• Chronic infection: Continuous virus shedding over time.

• Slow infection: Gradual disease progression over years.

Example: Herpes simplex virus establishes latency; HIV causes chronic infection.

Additional info: The notes above integrate foundational concepts from viral structure, replication, host interaction, and pathogenesis, as outlined in standard microbiology curricula (Chapters 5, 11, 25, 32, 33).