Viruses
🟢 Lite — Quick Review (1h–1d)
Rapid summary for last-minute revision before your exam.
Viruses — Key Facts for NEET PG
- Viruses: Obligate intracellular parasites; DNA or RNA genome; protein coat (capsid) ± envelope
- Capsid: Made of capsomeres; Functions: Protection, attachment, entry
- Baltimore Classification: Based on genome type and replication strategy (I-VII)
- Viral Replication: Adsorption → Penetration → Uncoating → Synthesis → Assembly → Release
- ⚡ Exam tip: DNA viruses replicate in nucleus (except poxviruses); RNA viruses replicate in cytoplasm (except influenza and HIV-retro)
🟡 Standard — Regular Study (2d–2mo)
Standard content for students with a few days to months.
Viruses — NEET PG Study Guide
Viral Structure
Components:
- Nucleic Acid: DNA or RNA; Single or double stranded; Linear or circular
- Capsid: Protein coat of capsomeres
- Envelope (some): Derived from host membrane; Contains viral glycoproteins (spikes)
Naked vs. Enveloped:
- Naked: Icosahedral symmetry; Resistant to detergents, drying, acid
- Enveloped: Pleomorphic or icosahedral; Sensitive to detergents, desiccation; Acquired from host membranes (nuclear, Golgi, plasma)
Viral Classification
Baltimore Classification:
| Class | Genome | Example |
|---|---|---|
| I | dsDNA | Adenovirus, Herpesvirus, Poxvirus |
| II | ssDNA (+) | Parvovirus |
| III | dsRNA | Reovirus, Rotavirus |
| IV | ssRNA (+) | Poliovirus, Hepatitis A, Flavivirus |
| V | ssRNA (-) | Influenza, Rabies, Measles |
| VI | ssRNA-RT (+) | HIV, HTLV |
| VII | dsDNA-RT | Hepatitis B |
NCE Exam Pattern
Common question types:
- Viral structure and classification
- Viral replication
- DNA vs RNA viruses
- Antiviral drugs and mechanisms
- Viral pathogenesis
🔴 Extended — Deep Study (3mo+)
Comprehensive coverage for students on a longer study timeline.
Viruses — Comprehensive NEET PG Notes
Detailed Theory
1. Viral Structure and Symmetry
Capsid Symmetry:
- Icosahedral: 20 triangular faces, 12 vertices; Efficient packing; Most viruses
- Helical: Rod or filament shaped; Tobacco mosaic virus (TMV)
- Complex: Poxviruses (brick-shaped), Bacteriophages (head-tail)
Viral Enzymes:
- Reverse transcriptase: HIV, HTLV, Hepatitis B
- Neuraminidase: Influenza
- Hemagglutinin: Influenza
- DNA polymerase: Herpesvirus
- RNA-dependent RNA polymerase: Influenza, Poliovirus
2. Viral Replication — Complete Cycle
1. Adsorption (Attachment):
- Specific binding of viral attachment proteins (VAP) to host cell receptors
- Examples:
- HIV: gp120 binds CD4 + CCR5/CXCR4
- EBV: gp350 binds CD21
- Influenza: Hemagglutinin binds sialic acid
- Determinant of host range and tissue tropism
2. Penetration (Entry):
- Naked viruses: Direct penetration (pore formation) or endocytosis
- Enveloped viruses:
- Fusion (envelope with cell membrane) — pH-dependent or independent
- Hemifusion (less common)
- Clathrin-mediated endocytosis for many viruses
3. Uncoating:
- Removal of capsid to release genome
- Naked viruses: Uncoat in cytoplasm or endosome (low pH triggers)
- Enveloped viruses: Fusion removes envelope; nucleocapsid enters cytoplasm
4. Synthesis (Replication and Expression):
-
DNA viruses (except Pox):
- Replicate in nucleus (host DNA polymerase)
- Early transcription → early proteins (replication enzymes)
- Late transcription → late proteins (structural proteins)
-
Poxviruses (cytoplasmic):
- Carry own DNA-dependent RNA polymerase
- Entire replication in cytoplasm
-
RNA viruses:
- (+)[sense] RNA: Acts as mRNA; genome replication via RNA-dependent RNA polymerase (RdRp)
- (-)[sense] RNA: Must first be transcribed to (+)[sense] mRNA by viral RdRp
- dsRNA: Each segment transcribed separately
- Retroviruses: RNA → DNA via reverse transcriptase
5. Assembly (Maturation):
- Self-assembly of capsomeres around genome
- Virion assembly complete
6. Release:
- Budding: Enveloped viruses exit via host membranes
- Sequential budding from different membranes
- Exocytosis: Non-enveloped viruses released by cell lysis
- Cell-to-cell fusion: Syncytia formation (HIV, HSV)
3. DNA Viruses
Poxviridae (largest, complex):
- Variola (Smallpox — eradicated)
- Vaccinia, Cowpox
- Molluscum contagiosum
- Monkeypox
- Key: Cytoplasmic replication, brick-shaped
Herpesviridae (dsDNA, icosahedral, enveloped):
- HHV-1/2: HSV-1 (oral), HSV-2 (genital)
- HHV-3: VZV (Chickenpox, Shingles)
- HHV-4: EBV (Mononucleosis, Burkitt’s lymphoma)
- HHV-5: CMV (Congenital infections, mononucleosis)
- HHV-6: Roseola infantum
- HHV-7: Pityriasis rosea
- HHV-8: Kaposi’s sarcoma (AIDS)
- Properties: Latency, nuclear replication, syncytia
Adenoviridae (dsDNA, naked, icosahedral):
- Respiratory infections, conjunctivitis, gastroenteritis
- Non-enveloped, icosahedral, fiber proteins for attachment
Parvoviridae (ssDNA, naked, icosahedral):
- Parvovirus B19: Erythema infectiosum (Fifth disease), aplastic crisis
- Smallest DNA virus
Hepadnaviridae (partially dsDNA, enveloped):
- Hepatitis B virus (HBV)
- Dane particle (complete virion)
- Reverse transcription during replication
- Pre-core antigen (HBeAg) marker
4. RNA Viruses
Picornaviridae (+ssRNA, naked):
- Poliovirus (paralytic polio)
- Rhinovirus (common cold, >100 serotypes)
- Hepatitis A virus
- Enterovirus 71 (hand-foot-mouth disease)
- Properties: Small RNA genome, icosahedral, no envelope
Paramyxoviridae (-ssRNA, enveloped):
- Paramyxovirus: Mumps, Parainfluenza
- Morbillivirus: Measles
- Pneumovirus: RSV
- Metapneumovirus: hMPV
- Properties: Hemagglutinin + neuraminidase, fusion protein, helical
Orthomyxoviridae (-ssRNA, segmented, enveloped):
- Influenza A, B, C (only A and B cause human epidemics)
- 8 segments of (-)[sense] RNA
- Hemagglutinin (attachment, entry) + Neuraminidase (release)
- Antigenic drift (point mutations) vs. shift (reassortment)
- Pandemic strains from reassortment
Rhabdoviridae (-ssRNA, bullet-shaped, enveloped):
- Rabies virus (Lyssavirus)
- Vesicular stomatitis virus
- Bullet-shaped, helical nucleocapsid
Filoviridae (-ssRNA, filamentous, enveloped):
- Ebola virus
- Marburg virus
- Hemorrhagic fevers
Flaviviridae (+ssRNA, enveloped):
- Dengue fever, Yellow fever, Zika, West Nile, Japanese encephalitis, HCV
- Transmitted by arthropods (except HCV)
- C, E envelope proteins
Coronaviridae (+ssRNA, enveloped, club-shaped spikes):
- Common cold (229E, NL63, OC43, HKU1)
- SARS-CoV, MERS-CoV, SARS-CoV-2
- Spike protein for attachment
- Replicates in cytoplasm
Togaviridae (+ssRNA, enveloped):
- Rubella virus (German measles)
- Alphavirus (Chikungunya, Eastern/Western equine encephalitis)
Retroviridae (diploid +ssRNA, enveloped):
- HIV-1, HIV-2 (Lentivirus)
- HTLV-1, HTLV-2 (Oncovirus)
- Reverse transcriptase → DNA → integration
- Integrase, protease enzymes
5. Prions and Viroids
Prions:
- Proteinaceous infectious particles
- No nucleic acid
- PrPSc (scrapie isoform) causes disease
- Transmissible spongiform encephalopathies (TSEs):
- Creutzfeldt-Jakob disease (CJD)
- Variant CJD (vCJD, BSE link)
- Kuru
- Fatal familial insomnia
- Slow incubation, progressive, fatal
Viroids:
- Small circular ssRNA (no protein coat)
- Plant pathogens
- Exception to central dogma (RNA can be infectious without protein)
6. Viral Pathogenesis
Transmission Routes:
- Respiratory (influenza, measles)
- Fecal-oral (rotavirus, poliovirus)
- Blood/body fluids (HIV, HBV, HCV)
- Sexual (HIV, HSV, HPV)
- Vector-borne (Flavivirus by mosquitoes)
- Zoonotic (Rabies, Hantavirus)
Tropism: Determined by viral receptors on host cells
Cell Tropism of HIV: CD4+ T cells, macrophages, dendritic cells (via CD4 + CCR5/CXCR4)
Cytopathic Effects (CPE):
- Cell rounding
- Syncytia formation (cell fusion)
- Intracytoplasmic inclusions (Negri bodies in rabies)
- Intranuclear inclusions (Cowdry type A in HSV)
- Cell lysis
- Transformation (EBV, HPV, HTLV)
Patterns of Infection:
- Acute: Influenza, common cold — resolve
- Persistent/Latent: HSV, HIV, HBV
- Chronic: HCV, chronic HBV
- Slow: Prion diseases, subacute sclerosing panencephalitis (SSPE) from measles
- Transforming: EBV, HPV, HTLV
7. Antiviral Agents
Anti-HIV:
- NRTIs: Zidovudine, Tenofovir, Lamivudine (block RT)
- NNRTIs: Efavirenz, Nevirapine (bind RT allosterically)
- Protease inhibitors: Ritonavir, Lopinavir (block protease)
- Integrase inhibitors: Raltegravir, Dolutegravir (block integration)
- Entry/Fusion inhibitors: Enfuvirtide (fusion), Maraviroc (CCR5 antagonist)
Anti-Influenza:
- Oseltamivir, Zanamivir (Neuraminidase inhibitors)
- Amantadine, Rimantadine (M2 ion channel blockers — resistant, not recommended)
Anti-Hepatitis:
- HBV: Nucleos(t)ide analogs (Entecavir, Tenofovir)
- HCV: Direct-acting antivirals (Sofosbuvir, Ledipasvir, Velpatasvir)
Anti-HSV/VZV:
- Acyclovir, Valacyclovir, Ganciclovir (Nucleoside analogs, require viral thymidine kinase)
- Foscarnet (binds DNA polymerase directly, no TK needed)
Anti-CMV:
- Ganciclovir (similar to acyclovir)
- Cidofovir (nucleotide analog)
- Foscarnet
8. Interferons and Innate Immunity
Interferons (IFNs):
- Type I (α, β): Induced by viral infection, act on all cells
- Type II (γ): Immune modulation, produced by T cells and NK cells
Mechanism:
- Infected cell releases IFN
- IFN binds receptors on neighboring cells
- Cells develop antiviral state (↑ PKR, ↑ OAS, ↑ RNase L)
- Protein synthesis inhibited
Clinical Use: IFN-α for Hepatitis B, C, some cancers
9. Viral Diagnostics
Microscopy:
- EM (negative staining) for rapid diagnosis
- Light microscopy for inclusion bodies
Culture:
- Cell lines (Vero, HeLa, MRC-5)
- Cytopathic effect (CPE)
- Hemadsorption (influenza, parainfluenza)
- Plaque assay
Serology:
- ELISA (antigen or antibody detection)
- Rapid antigen detection tests
- Neutralization tests
Molecular:
- PCR, RT-PCR
- Quantitative viral load
- Gene sequencing
10. Viral Oncogenesis
DNA Tumor Viruses:
- HPV: E6 (inactivates p53), E7 (inactivates Rb)
- EBV: Latent proteins (LMP1, EBNA)
- HBV: X protein (transactivator)
- MCPyV: Truncated large T antigen
RNA Tumor Viruses:
- HTLV-1: Tax protein (immortalization)
- HIV: Indirect (immune suppression)
Mechanisms:
- Inactivation of tumor suppressors (p53, Rb)
- Activation of oncogenes
- Immortalization
- Immune evasion
Practice Questions for NEET PG
- Describe the steps of viral replication.
- Compare and contrast DNA viruses and RNA viruses.
- Explain the mechanism of action of reverse transcriptase.
- Discuss the pathogenesis of HIV infection.
- What are the mechanisms of antiviral drug resistance?
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