Microbiology
Virus Replication
Attachment, entry, uncoating, replication, assembly, release — the viral life cycle and antiviral drug targets
Viruses are obligate intracellular parasites — they cannot replicate without hijacking host cell machinery. Life cycle: (1) attachment to specific host receptors (HIV gp120 → CD4 + CCR5/CXCR4; influenza HA → sialic acid; SARS-CoV-2 spike → ACE2); (2) entry by membrane fusion or endocytosis; (3) uncoating to release genome; (4) replication and gene expression — DNA viruses usually in nucleus, RNA viruses usually in cytoplasm using RNA-dependent RNA polymerase, retroviruses use reverse transcriptase to make DNA then integrate; (5) assembly of new virions from translated proteins and replicated genome; (6) release by lysis or budding, often acquiring envelope from host membrane. Each step is a drug target — neuraminidase inhibitors, protease inhibitors, polymerase inhibitors, integrase inhibitors, capsid inhibitors.
- Virus typesDNA, RNA (positive-sense, negative-sense, dsRNA), retrovirus
- Largest human virusesPoxviruses (~200 nm, ~200 kb DNA)
- Smallest human virusesParvovirus (~22 nm)
- Influenza mutation rate~10⁻³ per base per replication
- HIV reverse transcriptaseError-prone (~10⁻⁴) — drives resistance
- SARS-CoV-2 spike receptorHuman ACE2
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why virus replication matters
- Antiviral drug design. Each replication step is a target — protease, polymerase, integrase, neuraminidase inhibitors are clinical pillars.
- Vaccine development. Knowing receptor and entry mechanism guides antigen choice (HIV gp120, SARS-CoV-2 spike).
- HIV management. Combination ART suppresses replication to undetectable levels; U=U (undetectable = untransmittable).
- Pandemic preparedness. Mutation and reassortment patterns drive surveillance for novel influenza and coronaviruses.
- Cancer biology. Oncogenic viruses (HPV, EBV, HBV, HCV, HTLV-1, KSHV) cause ~15% of cancers worldwide.
- Diagnostic testing. PCR detects viral genome; antigen tests detect surface proteins; serology detects host antibody.
- Latency and reactivation. Immunosuppressed patients reactivate CMV, VZV, HSV, BK virus — clinical vigilance is critical.
Common misconceptions
- Viruses are alive. Viruses lack metabolism and reproduction independent of host cells — they straddle the boundary of life.
- Antibiotics treat colds and flu. Viral infections are unaffected; antibiotics are reserved for bacterial superinfection.
- Higher viral mutation rate is always advantageous. Most mutations are deleterious; high error rates impose error catastrophe limits on RNA virus genome size.
- All RNA viruses replicate in cytoplasm. Retroviruses and influenza require nuclear access for integration or transcription.
- Vaccines and antivirals are interchangeable. Vaccines prevent; antivirals treat. They serve complementary roles.
- HIV and AIDS are the same. HIV is the virus; AIDS is the late-stage immunodeficiency syndrome (CD4 <200 or AIDS-defining illness).
Frequently asked questions
How does HIV replicate?
HIV is a lentivirus (retrovirus). gp120 binds CD4 on T helper cells and macrophages, then engages CCR5 or CXCR4 coreceptor; gp41 mediates membrane fusion. Reverse transcriptase converts viral RNA to dsDNA (error-prone, drives resistance). Integrase inserts proviral DNA into host genome. Host RNA polymerase II transcribes new viral mRNA. Translation produces gag-pol polyprotein; HIV protease cleaves it. New virions bud from membrane. Antiretroviral therapy: NRTIs (tenofovir, emtricitabine), NNRTIs (efavirenz), integrase inhibitors (dolutegravir, bictegravir), protease inhibitors (darunavir), entry inhibitors (maraviroc).
How does influenza replicate and why does it mutate so rapidly?
Influenza is a segmented negative-sense RNA virus (8 segments). HA binds sialic acid on respiratory epithelium; endocytosed; M2 channel acidifies endosome triggering uncoating. RNA-dependent RNA polymerase (lacks proofreading → high mutation rate) replicates segments in nucleus. New segments and proteins assemble at plasma membrane; neuraminidase cleaves sialic acid, releasing virions. Antigenic drift: gradual point mutations causing seasonal flu and need for annual vaccine. Antigenic shift: reassortment between segments from animal and human strains creating pandemic strains (1918, 1957, 1968, 2009).
What's special about SARS-CoV-2?
Positive-sense single-stranded RNA virus. Spike protein binds ACE2 receptor on respiratory and other epithelia; TMPRSS2 protease activates spike. Genome translated directly as polyprotein, cleaved by viral proteases. RdRp replicates genome and makes subgenomic mRNAs. Spike protein is the major target of vaccines (mRNA, viral vector, protein subunit). Variants emerged through receptor-binding domain mutations enhancing transmission and immune escape (Alpha, Delta, Omicron). Paxlovid (nirmatrelvir/ritonavir) inhibits Mpro protease; remdesivir and molnupiravir target RdRp.
How do antiviral drugs work?
Each replication step is a target. Attachment/entry: maraviroc (CCR5), enfuvirtide (gp41 fusion), monoclonal antibodies. Uncoating: amantadine (influenza M2). Genome replication: nucleoside analogs (acyclovir for HSV/VZV thymidine kinase activated, tenofovir for HIV/HBV reverse transcriptase, sofosbuvir for HCV NS5B polymerase, remdesivir for SARS-CoV-2). Integration: dolutegravir, bictegravir (HIV integrase). Cleavage: protease inhibitors (HIV, HCV, SARS-CoV-2). Release: oseltamivir (influenza neuraminidase). Combination therapy in HIV and HCV prevents resistance.
What's the difference between latent and lytic infection?
Lytic: rapid replication, host cell death, virion release, often clinically apparent. Latent: viral genome persists in host cell (episome or integrated) without producing virions; can reactivate. Herpesviruses are master latent infectors: HSV-1/2 latent in trigeminal/sacral ganglia, reactivate as cold sores or genital lesions; VZV latent in dorsal root ganglia, reactivates as shingles. EBV latent in B cells (associated with Burkitt lymphoma, nasopharyngeal carcinoma). HIV integrates into host DNA — latent reservoir in resting T cells is the major barrier to cure.
How does the body fight viral infection?
Innate: type I interferons (IFN-α/β) produced by infected cells trigger antiviral state; pattern recognition receptors (TLRs, RIG-I, MDA5) detect viral nucleic acids; NK cells kill infected cells. Adaptive: CD8 T cells recognize viral peptides on MHC I and kill infected cells; CD4 T cells provide help; B cells produce neutralizing antibodies. Memory cells provide rapid response on re-exposure — basis of vaccination. Some viruses evade these defenses (HIV exhausts T cells; herpesviruses block MHC I; influenza NS1 inhibits IFN).
Why don't antibiotics work on viruses?
Antibiotics target bacterial structures absent in viruses or human cells: cell wall (penicillins, cephalosporins), bacterial ribosomes (aminoglycosides, macrolides), bacterial folate synthesis (sulfonamides), bacterial DNA gyrase (fluoroquinolones). Viruses use host machinery and lack these targets. Misuse of antibiotics for viral infections drives resistance and disrupts microbiome without benefit. Antiviral drugs require specific targets like viral polymerases, proteases, or integrases — discovered for only a fraction of human viruses.