Question

How do viruses reproduce?

Answer 1

Viruses cannot reproduce on their own — they need a living host cell and must hijack the host’s molecular machinery. The basic viral replication cycle follows a common set of steps, although details vary widely between virus types.

Typical steps in viral reproduction
- Attachment (adsorption): the virus binds to specific receptor molecules on the surface of a susceptible host cell.
- Entry (penetration): the virus or its genome enters the cell. Entry can be by fusion of an enveloped virus with the cell membrane, by endocytosis, or (for some bacteriophages) by injecting the genome through the cell wall.
- Uncoating: the viral capsid is removed and the viral genome is released into the appropriate cellular compartment (cytoplasm or nucleus).
- Genome replication: the viral genome is copied using viral and/or host enzymes. How this works depends on the type of viral genome:
- DNA viruses often use host DNA polymerases in the nucleus (some carry their own enzymes).
- Positive-sense RNA viruses (like poliovirus) can have their RNA directly translated as mRNA.
- Negative-sense RNA viruses (like influenza) must carry an RNA-dependent RNA polymerase to make a positive-sense copy first.
- Retroviruses (like HIV) reverse-transcribe their RNA into DNA and integrate it into the host genome.
- Transcription and translation: viral genes are transcribed (if needed) and translated by host ribosomes to make viral proteins (structural proteins, enzymes).
- Assembly: new viral genomes and capsid proteins assemble into new virus particles (virions). Some viruses form assembly sites or “viral factories” inside the cell.
- Maturation (if required): some virions undergo proteolytic cleavage or structural changes to become infectious.
- Release: new virions leave the cell. Non-enveloped viruses often lyse (burst) the cell; enveloped viruses typically bud from the cell membrane, acquiring a lipid envelope with embedded viral proteins.

Variations and special cases
- Lytic vs lysogenic (temperate) cycles: bacteriophages and some animal viruses can follow a lytic route (immediate replication and host destruction) or integrate their genome into the host genome as a prophage/provirus and remain latent for long periods before reactivation.
- Latency and persistence: some viruses (herpesviruses, HIV) can persist in host cells in latent or low-level persistent forms and reactivate later.
- Different replication locations: some viruses replicate in the nucleus (many DNA viruses), others in the cytoplasm (many RNA viruses). Larger DNA viruses (poxviruses) bring their own replication enzymes and replicate in the cytoplasm.
- Genetic change: RNA viruses often mutate rapidly because their polymerases lack proofreading; segmented viruses (like influenza) can reassort segments between strains, producing rapid changes.

Examples
- Bacteriophage T4: attaches to an E. coli cell, injects DNA, replicates, assembles, and lyses the cell (lytic).
- Lambda phage: can integrate into the E. coli genome and remain dormant (lysogenic) or enter the lytic cycle.
- Influenza virus: negative-sense segmented RNA; must carry its own polymerase and leaves cells by budding.
- HIV: a retrovirus that reverse-transcribes RNA into DNA and integrates into the host genome before producing new virions.

Summary
Viruses reproduce by entering host cells and using host machinery to copy their genomes and make viral proteins, assemble new particles, and release those particles to infect more cells. The exact mechanisms vary greatly depending on the virus’s genome type and structure.