Viruses as biological replicators
Viruses are acellular infectious entities that depend on host-cell machinery. Their genomes can be DNA or RNA, single- or double-stranded, linear, circular, or segmented, and can be enclosed by a protein capsid with or without a lipid envelope.
- Identify virion structures.
- Distinguish major genome strategies.
- Trace attachment, entry, uncoating, synthesis, assembly, and release.
- Compare lytic and latent/lysogenic strategies.
- Connect antiviral targets to viral life-cycle stages.
Virus ≠ bacterium
Viruses do not have ribosomes, independent energy metabolism, or the full cellular machinery needed to reproduce on their own. This is why antibacterial drugs generally do not work against viruses.
Virion explorer
Structure details
Envelope note
Enveloped viruses acquire lipid membrane from host-cell membranes during assembly or release. Viral proteins embedded in that envelope often mediate receptor binding and membrane fusion.
Genome explorer
Baltimore classification idea
The Baltimore system groups viruses by the relationship between their genome and the production of messenger RNA. The central question is: How does this genome produce mRNA that host ribosomes can translate?
| Group | Genome | Route to mRNA |
|---|---|---|
| I | dsDNA | DNA → mRNA |
| II | ssDNA | ssDNA → dsDNA → mRNA |
| III | dsRNA | Viral polymerase makes mRNA |
| IV | (+)ssRNA | Genome can function as mRNA |
| V | (−)ssRNA | Viral polymerase makes (+)mRNA |
| VI | ssRNA-RT | RNA → DNA intermediate → mRNA |
| VII | dsDNA-RT | DNA genome replicated through RNA intermediate |
Replication cycle stepper
Stage explanation
Lytic strategy
In a lytic cycle, viral gene expression and genome replication lead relatively directly to production of new virions and release, often with substantial damage to the host cell.
Latent / lysogenic strategy
Some viruses can persist without immediately producing large numbers of virions. A viral genome may remain episomal or become integrated into host DNA, depending on the virus. Later signals can reactivate productive replication.
Concept challenge
A viral genome persists in a cell for a prolonged period with little production of new virions. Which strategy is most consistent?
Host specificity
A virus must bind compatible host molecules and operate within a cellular environment that supports its replication. Receptor availability, intracellular cofactors, innate defenses, tissue accessibility, and temperature can all influence tropism.
Innate response
Cells can detect viral nucleic acids or replication-associated molecules through pattern-recognition receptors. Interferon signaling can induce an antiviral state and recruit additional immune functions.
Adaptive response
Antibodies can neutralize extracellular virions or block attachment, while cytotoxic T cells can recognize and eliminate infected cells presenting viral peptides.
Antigenic change
Mutation and, for some viruses, reassortment or recombination can alter viral antigens. The biological consequences depend on which regions change and how those changes affect fitness and immune recognition.
Antiviral target mapper
Why viral drugs are challenging
Viruses use host-cell machinery, so useful antiviral targets often need to exploit virus-specific enzymes, structures, or lifecycle steps without causing unacceptable damage to host cells. Different viral families present very different target sets.