Virus replication

The replication cycle of a virus is a coordinated sequence of events that begins with the attachment of the virion to a susceptible host cell. Attachment is mediated by specific interactions between viral surface proteins and host cell receptors, such as the binding of the SARS-CoV-2 spike protein to ACE2¹. Following entry, the viral genome is uncoated and, depending on the virus type, is replicated and transcribed using either viral or host enzymes. Newly synthesized genomes and proteins are assembled into progeny virions, which are then released by budding or cell lysis. Each step offers potential targets for antiviral intervention, and the overall cycle can be completed in hours to days depending on the virus and cell type.

Viruses exhibit remarkable diversity in their replication strategies, reflecting differences in genome type and replication site. DNA viruses, such as herpesviruses, typically replicate in the nucleus and use host DNA polymerases, whereas RNA viruses, such as influenza virus, replicate in the cytoplasm using viral RNA-dependent RNA polymerases. Retroviruses, like HIV, reverse-transcribe their RNA genome into DNA, which integrates into the host chromosome. Baltimore classification groups viruses into seven classes based on genome type and replication mechanism, providing a framework for understanding these strategies². This diversity influences the design of antiviral drugs, which often target virus-specific enzymes.

Virus replication is intimately linked to host cell machinery, and viruses have evolved numerous strategies to hijack cellular processes. For example, many viruses manipulate the host cell cycle, inhibit apoptosis, and subvert immune responses to create a favorable environment for replication. Understanding these interactions has led to the development of antiviral drugs that target viral enzymes, such as the protease inhibitors used against HIV and the polymerase inhibitors used against hepatitis C virus³. Additionally, host-directed therapies that modulate cellular factors are being explored as broad-spectrum antivirals. The study of virus replication also informs vaccine development, as live-attenuated vaccines rely on impaired replication.

Beyond the canonical steps, virus replication includes several lesser-known phenomena. For instance, some viruses exhibit "defective interfering particles" that lack essential genes and can only replicate with the help of a helper virus, thereby modulating the course of infection. Another intriguing aspect is the formation of replication compartments or "viral factories" within the host cell, which concentrate viral components and increase replication efficiency. Additionally, certain viruses, such as the hepatitis delta virus, use the replication machinery of other viruses (hepatitis B virus) to propagate, a phenomenon known as satellite virus replication. These aspects highlight the complexity and adaptability of viral replication strategies.

Baltimore classification

A system that groups viruses into seven classes based on their genome type (DNA or RNA, single- or double-stranded) and their replication strategy.

Defective interfering particles

Viral particles that contain a genome with deletions or mutations, rendering them unable to replicate without a helper virus; they can interfere with the replication of the standard virus.

Viral factories

Specialized intracellular compartments where viral replication and assembly are concentrated, often formed by remodeling host cell membranes.