Viral Replication, Entry, and Infection Outcomes

Mechanisms of Entry and Exit

The method of entry and exit depends heavily on whether the virus is "naked" (lacking an outer membrane) or "enveloped" (possessing a phospholipid bilayer derived from a host membrane).

Entry Mechanisms

• Direct Penetration: Common in bacteriophages (viruses that infect bacteria) and naked animal viruses. In bacteriophages like T4, the virus uses enzymes like lysozyme to create a hole in the cell wall and then "injects" its genome into the cytoplasm, leaving the empty capsid outside.
• Endocytosis: The host cell engulfs the entire virion in a vesicle.
• Membrane Fusion: Enveloped viruses can fuse their lipid envelope directly with the host's cytoplasmic membrane, allowing the nucleocapsid to enter the cell.

Exit Mechanisms

• Lysis: The host cell bursts, releasing a large number of virions at once. This is typical of virulent bacteriophages and naked animal viruses.
• Budding: Enveloped viruses exit the cell by pushing through the host membrane, "stealing" a piece of it to form their own envelope. This is a slower process that often allows the host cell to remain alive for a period, leading to persistent infections.

The Baltimore Classification: Replication Strategies

The Baltimore Scheme classifies viruses based on the relationship between their genome and the production of mRNA. Because all viruses must eventually produce mRNA to undergo translation at the host ribosome, their replication strategies vary significantly.

DNA Viruses

• dsDNA (Double-stranded DNA): These viruses, such as Herpesviruses, often replicate in the nucleus. They produce different classes of mRNA: immediate early (regulatory), delayed early (DNA polymerase), and late (structural).
• ssDNA (Single-stranded DNA): Examples include bacteriophages like $\phi$X174.
• DNA Tumor Viruses: Some viruses, like Polyomaviruses (e.g., SV40), can integrate their DNA into the host genome, leading to transformation—the conversion of a normal cell into a tumor cell.

RNA Viruses

• +ssRNA (Positive-sense): The genome acts directly as mRNA and can be immediately translated by host ribosomes. Examples include Poliovirus (which causes lysis) and Coronaviruses (which assemble in the Golgi complex).
• -ssRNA (Negative-sense): These viruses must first transcribe a positive strand using a viral enzyme called RNA-dependent RNA polymerase. Influenza virus is a notable example; its segmented genome allows for antigenic shift, where different strains reassort to create new hybrid viruses that evade immunity.
• dsRNA (Double-stranded RNA): Viruses like Reoviruses (e.g., rotavirus) carry their own enzymes to synthesize mRNA from their double-stranded genome.

Retroviruses

Retroviruses (like HIV) possess a +ssRNA genome but do not use it directly as mRNA. Instead, they use the enzyme reverse transcriptase to convert their RNA into complementary DNA (cDNA). This DNA is then integrated into the host chromosome by the enzyme integrase, becoming a permanent provirus.

Note on Hepatitis B: While Hepatitis B is a DNA virus, it is unique because it utilizes a reverse transcription strategy involving an RNA intermediate, placing it in a special category of the Baltimore scheme.

Infection Outcomes

A viral infection can result in several different clinical and cellular outcomes:

1. Virulent (Lytic) Infection: The virus replicates rapidly and kills the host cell through lysis.
2. Latent Infection: The viral genome enters a dormant state (as a prophage in bacteria or a provirus in animals) and is replicated along with the host DNA without producing new virions. The virus can reactivate later due to environmental stress.
3. Persistent Infection: Often caused by budding, the virus is released slowly, allowing the host cell to remain alive and continue producing virus for a long period.
4. Transformation: The virus induces the conversion of a normal cell into a tumor cell. This can happen if a virus inserts itself near a proto-oncogene, turning it into an oncogene, or if it disrupts a repressor gene that normally controls cell growth.