Viral Replication Cycles and Terminology

Exam Tip: The One-Step Growth Curve Viral replication is often visualized using a "one-step growth curve". When a single virion enters a cell, the measurable viral load drops to zero. Following the eclipse phase (genome replication and protein translation) and the maturation phase, there is a sudden, massive spike in viral count upon release. The total number of virions released per infected cell is known as the burst size.

Key Terminology: Eclipse, Latency, and Lysogeny

To describe the timing and state of an infection, virologists use specific terms:

• Eclipse Phase: The period during which the virus is actively replicating its genome and translating proteins, but no complete, infectious virions can yet be detected inside the cell.
• Maturation: The specific stage of packaging nucleic acids into the capsid.
• Latent Period: The total time encompassing both the eclipse and maturation phases.
• Lytic Cycle: A virulent pathway where the virus immediately undergoes synthesis, assembly, and release, resulting in the death of the host cell.
• Lysogeny: A strategy used by temperate viruses where the viral genome integrates into the host chromosome rather than killing the cell immediately.
  • In bacteria, the integrated DNA is called a prophage.
  • In animal cells, the integrated DNA is called a provirus.
  • This state can be reversed through induction, where environmental stressors cause the viral DNA to detach from the host genome and re-enter the lytic cycle.

Replication Strategies by Genome Type (The Baltimore Scheme)

The way a virus replicates depends heavily on whether its genome is DNA or RNA, and whether it is single-stranded (ss) or double-stranded (ds).

DNA Viruses

• dsDNA Viruses: These typically use the host's RNA polymerase for transcription and host DNA polymerase for replication.
• ssDNA Viruses: These must first be converted into dsDNA by host enzymes before they can be transcribed or replicated.

RNA Viruses

Because host cells do not naturally possess enzymes to copy RNA from an RNA template, most RNA viruses must carry or encode their own RNA-dependent RNA polymerase.

• dsRNA Viruses: Use viral RNA polymerase to synthesize mRNA (+ssRNA) from the double-stranded template.
• +ssRNA Viruses: The genome is "sense" orientation, meaning it can act directly as mRNA for immediate translation by host ribosomes. To replicate, they create a -ssRNA intermediate to serve as a template for new +ssRNA genomes.
• -ssRNA Viruses: The genome is "antisense" and cannot be translated directly. It must first be transcribed into +ssRNA using viral RNA-dependent RNA polymerase.

Retroviruses and Hepadnaviruses

These two classes are unique because they utilize reverse transcriptase, an enzyme that synthesizes DNA from an RNA template.

• Retroviruses (Class VI, e.g., HIV): These are enveloped viruses with two identical copies of ss(+)RNA. The enzyme reverse transcriptase performs three roles: reverse transcription (RNA $\rightarrow$ DNA), ribonuclease activity (degrading the original RNA strand), and DNA polymerase activity (synthesizing dsDNA from ssDNA). The enzyme integrase then inserts this DNA into the host genome to form a permanent provirus. Finally, protease is required for the maturation of new virions.
• Hepadnaviruses (Class VII, e.g., Hepatitis B): These possess unusual, partially double-stranded DNA genomes. They replicate through an RNA intermediate and use a unique reverse transcriptase that functions as a protein primer for DNA synthesis.

Summary Table: Viral Replication Outcomes