A virus is a microscopic infectious agent consisting of genetic material (DNA or RNA) encased in a protective protein shell called a capsid. This particle is inert until it successfully enters a living host cell. Once inside, the virus initiates a systematic takeover of the cell’s internal machinery. Its sole purpose is to hijack the cell’s resources—energy, enzymes, and building blocks—to manufacture thousands of copies of itself, converting the host cell into a virus-producing factory.
Unpacking the Viral Cargo
The first operation inside the host cell is uncoating, where the viral genome is released from its protective capsid. This separation is necessary so the genetic material is accessible for replication and protein production. The virus employs various strategies to shed its coat, often relying on existing host cell structures.
One common method involves host cell enzymes, specifically proteases, which chemically break down the protein shell. Another mechanism capitalizes on the naturally decreasing acidity (low pH) within cellular compartments called endosomes. As the virus is trafficked through the endosome, the change in pH triggers a shift in viral proteins, causing the capsid to destabilize and disassemble.
Once freed, the viral genetic material is targeted to the appropriate site for manufacturing. Most DNA viruses must transport their genome into the host cell nucleus, where the host’s genetic machinery resides. Conversely, most RNA viruses remain in the cytoplasm, as they do not require access to the nucleus for their replication.
Replication and Protein Synthesis
With the genome exposed, the cellular manufacturing phase begins, forcing the cell to produce viral components instead of its own proteins. The specific strategy depends on the type of nucleic acid the virus carries, but all viruses must ultimately produce messenger RNA (mRNA) that the host cell’s ribosomes can translate into viral proteins.
DNA Viruses
DNA viruses typically leverage the host cell’s DNA-dependent DNA polymerase enzymes to replicate their genome inside the nucleus. They also rely on the host’s RNA polymerase to transcribe their genes into viral mRNA. Viruses with larger genomes may encode their own polymerase enzymes to achieve greater independence from host cell processes.
RNA Viruses
RNA viruses face a unique challenge because host cells lack the necessary enzyme to copy an RNA template directly. These viruses must therefore carry or synthesize their own RNA-dependent RNA polymerase (RdRp).
- Positive-sense RNA viruses have a genome that can be immediately read as mRNA by host ribosomes to synthesize the RdRp enzyme.
- Negative-sense RNA viruses must first use a packaged RdRp to transcribe their genome into a positive-sense mRNA strand before proteins can be made.
Retroviruses
Retroviruses, such as HIV, utilize a specialized enzyme called reverse transcriptase. This enzyme converts the viral RNA genome into a double-stranded DNA copy, a process not naturally found in the host cell. The viral DNA then travels to the nucleus and integrates into the host cell’s chromosome, forming a provirus. Viral mRNA is translated by the host’s ribosomes to produce structural proteins (for the new virus shell) and functional proteins (enzymes required for replication).
Assembly of Progeny Virions
Once all the necessary building blocks are manufactured, the final stage is the physical construction of new virus particles, known as virions. Assembly requires the precise packaging of a newly replicated viral genome into a newly synthesized protein capsid. For many viruses, this is a process of self-assembly, where protein subunits spontaneously arrange themselves around the nucleic acid.
The viral genome contains a specific packaging signal recognized by structural proteins, ensuring that only viral nucleic acid is incorporated. This selective packaging is crucial for generating infectious progeny. For enveloped viruses, structural proteins and genetic material gather near a cellular membrane (such as the plasma membrane or Golgi apparatus). Specialized viral proteins are inserted into the host cell membrane at the assembly site in preparation for the final exit.
Release and Host Cell Outcome
The final act of the viral life cycle is the release of new virions, which determines the fate of the infected host cell. Viruses employ two main exit strategies, depending on whether they are enveloped or non-enveloped.
Lysis (Non-Enveloped Viruses)
Non-enveloped viruses typically utilize lysis, resulting in the immediate destruction and death of the host cell. Lysis involves viral proteins that weaken and rupture the host cell membrane, allowing accumulated virions to spill out. This explosive release effectively spreads a large number of new infectious particles.
Budding (Enveloped Viruses)
Enveloped viruses generally exit by budding. Budding occurs when the assembled nucleocapsid pushes against a cellular membrane modified with viral proteins. The virion pinches off, acquiring a piece of the host’s lipid membrane as its outer envelope. This method often allows the host cell to remain intact and viable, enabling it to continue producing and releasing new virus particles over time.
Latency
A third outcome is latency or persistence, occurring when some viruses (like herpesviruses) integrate their genome into the host’s DNA or maintain it separately without initiating replication and release. In this dormant state, the cell is not immediately killed, and the viral genome is replicated along with the host’s own, waiting for a trigger to reactivate the cycle.