Comprehensive Virology: Virus Structure, Classification, and Lifecycle

Who first identified viruses as infectious agents, and who first defined them as a new class of pathogen?

Dmitri Ivanovsky (1892, Russian) first identified a filterable infectious agent while investigating Tobacco Mosaic Disease — he mistakenly believed it was a toxin. Dutch microbiologist Martinus Beijerinck (1898) was first to use the word "virus" to describe a new class of infectious agent that could only replicate inside living cells, could not be cultured like bacteria, and whose effects increased over time — naming it Tobacco Mosaic Virus (TMV).

What was the first animal virus to be identified by filtration, and what disease does it cause?

An aphthovirus — the causative agent of foot-and-mouth disease (FMD). It was filtered in a way that separated it from bacteria, confirming it was a distinct, smaller infectious agent.

What is the origin of the word "vaccination" and what did Jenner's experiment demonstrate?

Vaccination comes from the Latin word "vacca" (cow). In 1796, Edward Jenner inoculated 8-year-old James Phipps with cowpox (from a milkmaid's blister) and showed it provided cross-protection against smallpox (Variola virus). Jenner is known as the "Father of Vaccination." Smallpox (Variola Major) caused blisters, fever, blindness, and death in >30% of cases; Variola Minor killed <1%.

What is the taxonomic hierarchy for classifying viruses and what suffixes are used?

Group (based on nucleic acid type) → Family (suffix: -viridae) → Genus (suffix: -virus) → Species (name). Example: Herpesviridae (family) → Simplexvirus (genus) → Herpes simplex virus 1 (species).

What is host affinity in virology, and give examples of both specific and broad host range viruses?

Host affinity refers to which hosts and which cell types a virus can infect. Specific (narrow range): HIV targets CD4⁺ T cells and macrophages; Hepatitis viruses target liver cells; Rabies virus targets neurons; HPV primarily targets humans. Broad (wide range): Influenza A viruses infect birds, pigs, and humans. Viruses must infect and replicate inside host cells — highly specific interactions govern this.

What are the three main host range categories for viruses, and give two examples from each?

Plant viruses: Tobacco Mosaic Virus (TMV), Tomato Spotted Wilt Virus. Bacteriophages (bacterial viruses): Phage T4, Phage λ. Animal viruses — DNA: Herpesviruses (cold sores, chickenpox), Adenoviruses (respiratory infections, pink eye), Poxviruses (smallpox), Papillomaviruses (warts, some cancers). Animal viruses — RNA: Poliovirus, Hepatitis C, Dengue, Zika, SARS-CoV-2, Rhinovirus, Influenza, HIV, Rabies.

What are the four methods used to culture viruses in the laboratory?

1) Bacteria culture (for bacteriophages). 2) Chick embryos (virus is inoculated into fertile chicken eggs — used for influenza vaccine production). 3) Cell/tissue culture (most common for animal viruses — primary cells, cell lines). 4) Purification by size or charge. Key assays used: Hemagglutination assay (detects viruses that bind red blood cells) and Plaque assay (quantifies infectious virions).

What does "acellular" mean in the context of viruses, and what are the two phases of a virus's existence?

Acellular means viruses are NOT cells — they have no metabolism, ribosomes, or ability to independently reproduce. Two phases: 1) Extracellular phase: the virion — the complete, infectious particle outside a host cell; metabolically inert. 2) Intracellular phase: inside the host cell — the virus hijacks host machinery to replicate. Viruses are obligate intracellular parasites.

What is a virion and what are its essential structural components?

A virion is the complete, mature, extracellular viral particle. Essential components: 1) Nucleic acid (NA): the viral genome — DNA or RNA. 2) Capsid: protein coat surrounding and protecting the NA. Together = nucleocapsid. Optional: 3) Envelope: lipid bilayer (± glycoprotein spikes) surrounding the nucleocapsid in enveloped viruses.

What is the size range of viruses and why does this matter clinically?

Viruses range from 10-400 nm in diameter — far smaller than bacteria (~1000 nm) and even most large proteins. This means they pass through bacterial filters (how they were first distinguished from bacteria), cannot be seen by light microscopy (require electron microscopy), and are too small to be eliminated by many standard sterilization filters.

What are the three types of capsid symmetry, give an example of each, and state which is most common in animal viruses?

1) Helical: capsomeres arranged in a helix around the NA — often seen in RNA viruses (e.g., Rabies, TMV, Influenza). 2) Polyhedral (icosahedral): most common in animal viruses — 20 triangular faces, highly symmetric (e.g., HPV, Adenovirus, Poliovirus, Hepatitis A). 3) Complex: combination structure, e.g., bacteriophages with an icosahedral head + helical tail (e.g., T4 phage).

What are capsomeres?

Capsomeres are the individual protein subunits that assemble to form the capsid. Multiple capsomeres come together symmetrically to build the full capsid structure around the viral nucleic acid. Their arrangement determines capsid symmetry (helical vs. polyhedral).

What are the two categories of viruses based on envelope presence, and how does this affect susceptibility to disinfectants?

Enveloped viruses: have a lipid bilayer membrane surrounding the nucleocapsid (e.g., HIV, Herpes, Influenza, Rabies, Hepatitis B). Naked (non-enveloped) viruses: no lipid envelope, only a protein capsid (e.g., Adenovirus, Poliovirus, Rotavirus, Hepatitis A). Enveloped viruses are LESS stable in the environment and more susceptible to disinfectants (detergents, alcohol) because the lipid envelope is easily disrupted. Naked viruses are MORE environmentally stable and harder to inactivate.

What is the viral envelope, where does it come from, and what are its components?

The viral envelope is a phospholipid bilayer that surrounds the nucleocapsid of enveloped viruses. Source depends on the virus — it is derived from HOST membranes: plasma membrane (HIV), nuclear membrane (Herpes), or endoplasmic reticulum (Hepatitis B). Components: phospholipid bilayer, embedded viral membrane proteins, and often glycoprotein spikes. Examples of enveloped viruses: Influenza, Rabies, Herpes, HIV.

What are envelope glycoprotein spikes and what are their two key functional roles?

Glycoprotein spikes are carbohydrate-linked protein projections on the outer surface of the viral envelope. Two roles: 1) Docking/Attachment: bind specific receptors on the host cell surface (e.g., HIV gp120 binds CD4; Influenza hemagglutinin (HA) binds sialic acid). 2) Fusion: mediate fusion of the viral envelope with the host cell membrane, allowing genome entry (e.g., Influenza HA2, HIV gp41).

What types of nucleic acid can viral genomes be, and what are the possible configurations?

Viral genomes can be DNA OR RNA (never both). Configuration options: Shape — circular or linear. Strand number — one or more segments. Strandedness — single-stranded (ss) or double-stranded (ds). Polarity (if RNA) — positive sense (+, same sequence as mRNA) or negative sense (−, complementary to mRNA). No other organism uses RNA as heritable genetic material — this is unique to viruses.

Give the Baltimore classification of viruses with genome type and an example for each class.

Class I (dsDNA): Smallpox, Herpes, Papillomavirus, Adenovirus. Class II (ssDNA): Parvovirus. Class III (dsRNA): Rotavirus. Class IV (+ssRNA): Poliovirus, Rhinovirus, Coronavirus. Class V (−ssRNA): Measles, Mumps, Rabies, Influenza. Class VI (Retroviruses, +ssRNA → DNA): HIV. Class VII (dsDNA with RNA intermediate): Hepatitis B.

What are the four steps of the bacteriophage lytic cycle?

1) Attachment and Entry: phage tail fibers bind specific receptors on the bacterial cell wall; genome is injected. 2) Synthesis: phage takes over host biosynthetic machinery to make phage nucleic acid and proteins. 3) Assembly: phage components self-assemble into new virions (heads, tails, tail fibers). 4) Lysis: phage lysozyme degrades the bacterial cell wall → cell bursts, releasing new phage particles (burst size = ~100-300 for T4).

What are the four steps of the bacteriophage lysogenic cycle and when does it re-enter lytic?

1) Attachment and Entry: phage injects genome into host. 2) Integration: phage NA integrates into the bacterial chromosome → becomes a prophage. 3) Binary fission: prophage is replicated along with host DNA and passed to daughter cells (vertical transmission). 4) Induction: environmental stressors (UV, DNA damage) trigger prophage excision → re-enters lytic cycle → Synthesis → Assembly → Lysis.

Describe the four steps of T4 phage attachment to E. coli in order.

I) Tail fibers of T4 make initial reversible contact with LPS on the host cell surface. II) The baseplate undergoes a large conformational change from dome-shaped to star-shaped → irreversible attachment to the outer membrane. III) The tail sheath contracts from its extended state (driven by ATP). IV) The rigid tail tube pierces the outer membrane → T4 genome is injected directly into the cytoplasm (only the DNA enters; the capsid remains outside).

What is the difference between generalized and specialized transduction?

Generalized transduction: ANY fragment of host bacterial DNA can be accidentally packaged into a phage capsid instead of phage DNA (packaging is non-specific); the transducing particle can transfer any gene to a new host (e.g., P1 phage in E. coli). Specialized transduction: ONLY genes adjacent to the prophage integration site can be transferred; occurs when prophage excision is imprecise and takes flanking bacterial genes with it (e.g., λ phage transferring gal or bio genes).

What are the three key features visible on a bacteriophage one-step growth curve?

1) Latent (eclipse) period: no phage detectable outside cells — phage are disassembled, genome replicating. 2) Rise period (burst): rapid increase in phage numbers as cells lyse and release virions. 3) Plateau: maximum phage titer reached — all infected cells have lysed. Burst size = total phage released ÷ number of originally infected bacteria.

How does animal virus entry differ from bacteriophage entry, and what are the three entry mechanisms for animal viruses?

Bacteriophages inject only their genome — the capsid stays outside. Animal viruses: the ENTIRE virion (or nucleocapsid) enters the host cell. Three mechanisms: 1) Direct Penetration: naked (non-enveloped) viruses — capsid interacts with membrane, genome is released directly into cytoplasm (e.g., Adenovirus, Poliovirus). 2) Membrane Fusion: enveloped viruses fuse their lipid envelope with the host plasma membrane, releasing nucleocapsid into cytoplasm (e.g., HIV, Paramyxovirus). 3) Endocytosis: virion is taken up into an endosome (e.g., Influenza, Adenovirus) — low pH triggers conformational changes for uncoating.

How does Influenza virus use the low pH of the endosome for uncoating?

After endocytosis, the acidic pH (~5) of the endosome activates the M2 ion channel in the Influenza virion. M2 pumps H⁺ ions into the viral interior, acidifying it and disrupting protein-protein interactions holding the viral RNP (ribonucleoprotein) complex together. This causes uncoating — the genome is released into the cytoplasm. This is the mechanism targeted by amantadine and rimantadine (M2 inhibitors — antiviral drugs).

Summarize nucleic acid synthesis strategies for each major virus class.

dsDNA: standard semiconservative DNA replication (usually in nucleus). ssDNA: host enzymes first make a complementary strand → then normal dsDNA replication. dsRNA: + strand = mRNA → used as template for new copies. +ssRNA: genome IS the mRNA → used directly; complementary strand made as template for new genomes. −ssRNA: viral RNA-dependent RNA polymerase (RDRP) makes + strand → serves as mRNA AND template for new −ss genomes. Retroviruses (+ssRNA): reverse transcriptase makes dsDNA from RNA template → DNA integrates into host genome → host RNA Pol transcribes new +ssRNA genomes.

Where does viral assembly occur for DNA vs. RNA viruses, and what happens with Herpesvirus?

DNA viruses: typically assemble in the NUCLEUS (where DNA replication occurs), then move to the cytoplasm. RNA viruses: typically assemble in the CYTOPLASM (where RNA replication occurs). Herpesvirus (special case — enveloped DNA virus): capsid assembles in the nucleus → buds through the nuclear membrane gaining its envelope → enters the perinuclear space → buds into Golgi-derived vesicles → vesicles fuse with plasma membrane → virions are exocytosed.

What are the four mechanisms of viral release from host cells?

1) Budding: virus acquires a membrane from the host cell as it exits (forms the envelope); cell is not immediately killed (e.g., HIV, Influenza). 2) Exocytosis: virus packaged in vesicles is released by fusion with the plasma membrane (e.g., non-lytic release of some enveloped viruses). 3) Cell Lysis: non-enveloped (naked) viruses accumulate in the cell and then burst the cell to release thousands of virions; cell is killed (e.g., Poliovirus, Adenovirus). 4) Latency: virus enters a dormant state, not actively released; can reactivate later (e.g., HSV, HIV).

What is the difference between acute, latent, and persistent viral infections, with an example of each?

Acute infection: short-lived, resolves quickly and naturally, few lasting effects (e.g., Influenza, common cold/Rhinovirus). Latent infection: virus remains in the body after initial infection, integrated into host DNA or maintained episomally, not actively replicating, not detectable during dormancy but can reactivate (e.g., Herpes Simplex Virus — cold sores/shingles recur under stress). Persistent/Chronic infection: virus continuously present and replicating at low levels long-term (e.g., Hepatitis B, HIV). Slow infection: very long incubation, progressive disease (e.g., prions/CJD, HIV progressing to AIDS).

What are oncogenes, and how do viruses cause cancer?

Oncogenes are genes that, when mutated or abnormally activated, can drive uncontrolled cell proliferation (cancer). Viruses can cause cancer by: 1) Mutation: insertional mutagenesis — viral DNA integrates near a proto-oncogene, activating it. 2) Transduction: virus captures and delivers oncogenes to new cells. 3) Expressing viral proteins that: block p53 (which normally induces growth arrest for DNA repair) or block Rb (which normally induces apoptosis in abnormal cells). DNA tumor viruses: Adenovirus, Herpesvirus, Poxvirus, Papovavirus (HPV), Hepadnavirus (Hep B). RNA tumor viruses: Retroviruses (HIV, HTLV).

What are the main classes of antiviral drugs and their mechanisms?

1) Attachment antagonists: block viral attachment molecules on host cell (prevent docking). 2) Uncoating inhibitors: neutralize acidic endosome environment OR directly block uncoating. Examples: Amantadine and Rimantadine — block M2 ion channel of Influenza, preventing acidification and uncoating. Arildone — blocks picornavirus replication by inhibiting capsid uncoating in the cytoplasm. 3) DNA/RNA synthesis inhibitors: require phosphorylation activation by VIRAL kinases (selective — only activated in infected cells). Acyclovir: activated by HSV thymidine kinase → inhibits viral DNA polymerase → terminates chain elongation. Ganciclovir: similar mechanism, used for CMV.

What are prions, what is their structure, and how do they differ from all other infectious agents?

Prions (PrPSc) are infectious proteins — they contain NO nucleic acid (no DNA or RNA), which is unique among all known infectious agents. Normal form: PrPC (cellular) — soluble, predominantly α-helical. Infectious misfolded form: PrPSc (scrapie) — insoluble, predominantly β-pleated sheet structure, resistant to proteases, heat, and most standard sterilization methods. PrPSc acts as a template that converts normal PrPC into the misfolded PrPSc form in a self-propagating chain reaction. Accumulation causes: protein fibrils, vacuoles in nervous tissue (spongiform appearance), and neuronal death. There is NO nucleic acid involved — prions replicate by protein conformational conversion alone.

Name the four major prion diseases, which organism they affect, and key features of each.

1) BSE (Bovine Spongiform Encephalopathy/"Mad Cow Disease"): cattle — incurable, fatal, caused by consumption of contaminated meat-and-bone meal. 2) Scrapie: sheep and goats — causes wool loss, pinkish skin, poor health; first described prion disease. 3) CJD (Creutzfeldt-Jakob Disease): humans — fatal neurodegenerative disease, severe brain damage; 90% sporadic, 10% hereditary; fatal within ~1 year of onset. Discovered by Stanley B. Prusiner (1997 Nobel Prize). 4) Kuru: humans — historically in people of Papua New Guinea; transmitted via ritualistic cannibalism; caused tremors, loss of coordination; always fatal. Also: CWD (Chronic Wasting Disease) affects deer, elk, moose — severe weight loss, drooling, stumbling.

What is the neuropathological hallmark of prion disease and how does it progress?

Normal brain tissue appears dense, uniform with no vacuoles and normal architecture. As PrPSc accumulates in the CNS: small vacuoles form in neurons → progresses to large vacuoles and spongiform (sponge-like) appearance → extensive vacuolation with PrPSc deposition and neuronal loss. This "spongiform encephalopathy" pattern — the Swiss cheese appearance of the brain — is the defining microscopic hallmark of all prion diseases and is remarkably similar regardless of which prion disease is present.

What is the role of chaperones (CLIPS and ERAD) in prion disease pathology?

CLIPS (Chaperones Linked to Protein Synthesis): normally assist in correctly folding newly synthesized PrPC. In prion disease, when misfolded PrPSc is present, it overwhelms the chaperone system. ERAD (Endoplasmic Reticulum-Associated Protein Degradation): normally degrades misfolded proteins before they cause harm. In prion disease, PrPSc oligomers are highly ordered and insoluble, evading ERAD, and instead stabilize as oligomers. These highly ordered PrPSc oligomers incorporate endogenous PrPC, growing in size and propagating the misfolded conformation — a self-amplifying pathogenic process.

What viral or prion-associated conditions have been proposed or confirmed beyond classical infectious disease?

Confirmed viral associations: Hepatitis (HBV, HCV), SLE/Lupus (linked to EBV — Epstein-Barr Virus), Neoplasia/cancer (HPV, HTLV, HBV). Proposed/investigated associations: Alzheimer's Disease (ALZ) — prion-like propagation of misfolded amyloid-β; Multiple Sclerosis (MS) — EBV implicated; Parkinson's Disease — α-synuclein has prion-like misfolding properties; ALS (Amyotrophic Lateral Sclerosis) — TDP-43 aggregates behave prion-like; Degenerative Myelopathy (DM); Rheumatoid Arthritis (RA). These represent an emerging understanding that protein misfolding and viral triggers may underlie many neurodegenerative and autoimmune conditions.

What is the formal definition of a virus?

A sub-microscopic entity consisting of nucleic acid surrounded by a protein coat, capable of replication ONLY within the living cells of bacteria, animals, or plants. Also described as an "obligate intracellular parasite."

Why are viruses called "obligate intracellular parasites"?

Because viruses CANNOT replicate on their own — they have no ribosomes, no metabolism, and no energy-generating systems. They are entirely dependent on a living host cell to provide ribosomes, energy, metabolic intermediates, and raw materials for genome replication and protein synthesis. Outside a living host cell, viruses are completely inert.

What is Problem #1 that all viruses must solve during replication?

All viruses must replicate their genome AND direct the synthesis of viral proteins. Genome replication and mRNA transcription may be done by viral enzymes, cellular enzymes, or both — but translation of proteins is ALWAYS mediated by CELLULAR ribosomes. Viruses cannot translate proteins independently.

What is Problem #2 that all viruses must solve, and why does it create a challenge?

Viruses benefit from having a small genome (easier to replicate quickly, less energy cost, smaller virion). However, a small genome must encode ALL the information needed for replication, capsid proteins, enzymes, and immune evasion. This creates selective pressure to compress as much genetic information as possible into as few nucleotides as possible.

What is the core principle underlying the Baltimore Classification System?

All viruses, regardless of genome type, must produce mRNA (+sense) that is then translated by cellular ribosomes. The Baltimore system classifies viruses based on the nature and polarity of their genome AND the unique pathway each virus uses to get from its genome to mRNA. There are 7 classes.

Who proposed the Baltimore Classification and when?

David Baltimore proposed the classification system in 1971. It groups viruses into 7 classes based on their genome type (DNA or RNA), strandedness (ss or ds), polarity (+ or −), and the molecular pathway used to produce mRNA.

What is a key advantage and a key disadvantage of DNA viruses regarding replication?

Advantage: DNA viruses can use polymerases ALREADY PRESENT in the host cell nucleus — they do not necessarily need to encode or package their own polymerases, keeping genome size down. Disadvantage: they are more dependent on the cell cycle status of the host cell (host DNA polymerases are only active during S phase), AND most must transport their genome into the nucleus to replicate.

What is a key advantage and a key disadvantage of RNA viruses regarding replication?

Advantage: RNA viruses are more independent of the host cell — they can replicate entirely in the CYTOPLASM and do not need to enter the nucleus or wait for specific cell cycle stages. Disadvantage: they must encode AND package their own RNA-dependent RNA polymerase (RdRp). These polymerases are inherently ERROR-PRONE (no proofreading) → high mutation rates. RNA is also less chemically stable than DNA.

Describe Baltimore Class I viruses: genome type, replication location, and the two subgroups.

Class I = double-stranded DNA (dsDNA). Two subgroups: A) Nuclear replication — virus depends heavily on cellular transcription/replication factors (e.g., Herpesviruses, Adenoviruses, Papillomaviruses). B) Cytoplasmic replication — virus has evolved/acquired ALL necessary factors for transcription and genome replication in the cytoplasm; does NOT enter the nucleus (e.g., Poxviridae — Smallpox). Poxviruses are the main exception to the rule that DNA viruses replicate in the nucleus.

Describe Baltimore Class II viruses: genome type, replication process, and location.

Class II = single-stranded DNA (ssDNA). Replication occurs in the NUCLEUS. The ssDNA genome must first be converted to a double-stranded intermediate (dsDNA replicative form) using host enzymes. This dsDNA intermediate then serves as the template for synthesis of new single-stranded progeny DNA genomes. Example: Parvovirus.

Describe Baltimore Class III viruses: genome type and key feature of their genome.

Class III = double-stranded RNA (dsRNA). These viruses have SEGMENTED genomes — the genome is divided into multiple separate RNA segments. Each segment is transcribed SEPARATELY by the viral RNA-dependent RNA polymerase to produce individual monocistronic mRNAs (one mRNA per segment, one protein per mRNA). Example: Rotavirus (11 segments).

Why must Class III (dsRNA) viruses package their own RNA polymerase inside the virion?

Because host cells have NO dsRNA-dependent RNA polymerase — the host cannot transcribe the dsRNA genome. Therefore, the viral RdRp must be PACKAGED inside the virion and introduced into the host cell upon infection to immediately begin transcribing the genome into mRNAs before any viral proteins are made.

Describe Baltimore Class IV viruses: genome type and the two subgroups based on mRNA strategy.

Class IV = single-stranded positive-sense RNA (+ssRNA). The genome itself IS the mRNA — it can be directly translated by host ribosomes upon cell entry. Two subgroups: A) Polycistronic mRNA — genome is translated as one long polyprotein that is then CLEAVED by viral proteases into individual functional proteins (e.g., Poliovirus, Picornaviruses). B) Complex transcription — uses subgenomic mRNAs, ribosomal frameshifting, and proteolytic processing (e.g., Coronaviruses).

Why is a +ssRNA genome considered the simplest genome design?

Because the +ssRNA genome is directly equivalent to mRNA — it can be immediately translated by host ribosomes the moment it enters the cytoplasm, BEFORE any viral proteins are made. No intermediate steps (transcription into mRNA) are required. This makes these viruses highly efficient and fast to initiate an infection.

Describe Baltimore Class V viruses: genome type and the two subgroups.

Class V = single-stranded negative-sense RNA (−ssRNA). The genome is complementary to mRNA — it CANNOT be translated directly; it must first be transcribed into +sense mRNA by the viral RdRp. Subgroup A) Nonsegmented genome: RdRp transcribes the (−) genome into multiple monocistronic mRNAs → individual viral proteins made; then a full-length (+) copy is made as template for new (−) genomes (e.g., Rabies, Measles, Mumps). Subgroup B) Segmented genome: replication occurs in the NUCLEUS; RdRp makes one monocistronic mRNA per genome segment (e.g., Influenza — 8 segments).

Why must Class V (−ssRNA) viruses package their own RdRp inside the virion?

The (−) sense genome cannot be translated — it is the wrong polarity. Host cells have no RNA-dependent RNA polymerase to transcribe it. Therefore the viral RdRp MUST be pre-packaged inside the virion and injected into the cell along with the genome so it can immediately begin making +sense mRNAs before any viral protein is synthesized.

Describe Baltimore Class VI viruses: genome type, unique feature, and replication pathway.

Class VI = Retroviruses — single-stranded positive-sense RNA (+ssRNA) with a DNA intermediate. UNIQUE: the (+ssRNA genome does NOT serve directly as mRNA. Instead it is used as template for REVERSE TRANSCRIPTION into DNA by reverse transcriptase (packaged in virion). The resulting dsDNA integrates into the host genome (provirus). The host RNA polymerase then transcribes the integrated DNA to make viral mRNAs and new (+ssRNA genome copies. These are the ONLY viruses that use RNA→DNA→RNA information flow. Example: HIV. The genome is DIPLOID — two identical copies of +ssRNA per virion.

What does it mean that retroviruses have a "diploid" genome?

Retroviral virions contain TWO identical copies of the +ssRNA genome (rather than one). This provides a backup if one copy is damaged, and also allows recombination between the two copies during reverse transcription, increasing genetic diversity. This is unique among all viruses — no other virus class carries two identical genome copies.

Describe Baltimore Class VII viruses: genome type, key structural feature, and replication strategy.

Class VII = dsDNA with an RNA intermediate (e.g., Hepatitis B virus). Genome is double-stranded but GAPPED — one strand is incomplete. Upon infection, the gap is filled in to form a covalently closed circular DNA (cccDNA) in the nucleus. Host RNA polymerase transcribes cccDNA → viral mRNAs AND a pregenomic RNA (pgRNA). The viral reverse transcriptase uses pgRNA as template to synthesize new DNA genomes. Unique: uses both transcription (DNA→RNA) AND reverse transcription (RNA→DNA) in its replication cycle.

What is cccDNA in Hepatitis B and why is it clinically significant?

cccDNA (covalently closed circular DNA) is the fully repaired, circular form of the Hepatitis B genome formed in the nucleus after the gapped dsDNA is filled in. It serves as the transcriptional template for all viral mRNAs and the pregenomic RNA. Clinically significant because cccDNA is extremely stable, persists in hepatocyte nuclei for years, is not targeted by most current antivirals, and is the main reason HBV infection is so difficult to cure — even when the virus is suppressed, cccDNA remains as a reservoir for reactivation.

What is an overlapping open reading frame (ORF) and why do viruses use it?

An overlapping ORF occurs when two or more protein-coding sequences share the same stretch of nucleotides but are read in different reading frames or from different start codons on the same or opposite strand. This allows one segment of nucleic acid to encode multiple different proteins simultaneously — a major way viruses increase information density without increasing genome size. Example: φX174 phage gene A encodes both A and A* proteins from overlapping frames.

What is a polyprotein and how do viruses use it to solve the small genome problem?

A polyprotein is a single large protein translated from a polycistronic mRNA (or directly from the genome in Class IV viruses). After translation, viral proteases cleave the polyprotein at specific sites to release multiple individual functional proteins. This strategy allows one long ORF to encode many proteins — maximizing the number of proteins produced from a minimal amount of RNA. Example: Poliovirus translates its entire genome as one polyprotein → cleaved into 11 proteins.

What is ribosomal frameshifting and which virus class commonly uses it?

Ribosomal frameshifting occurs when the ribosome "slips" by one or two nucleotides during translation at a specific signal sequence, changing the reading frame and continuing translation in the new frame. This produces two different proteins from the same mRNA — one from the original frame and one from the shifted frame. Used by: Class IV (retroviruses like HIV use −1 frameshifting to produce Gag-Pol fusion protein) and Coronaviruses. Allows one RNA region to encode two proteins without gene duplication.

What is an Internal Ribosomal Entry Site (IRES) and which viruses use it?

An IRES is a structured RNA sequence within the 5' UTR of some viral mRNAs that allows ribosomes to initiate translation INTERNALLY — directly at an AUG codon deep within the mRNA — without scanning from the 5' cap. This allows cap-independent translation, which is useful when viruses suppress host cell cap-dependent translation. Examples: Picornaviruses (Poliovirus, Hepatitis A), Hepatitis C virus.

What is mRNA splicing in the context of viral genomes?

Splicing is the removal of intron sequences from a pre-mRNA to produce a shorter, mature mRNA. Some DNA viruses (that replicate in the nucleus) can exploit the host cell's spliceosome machinery to splice their pre-mRNAs, generating multiple different proteins from a single genomic sequence. This increases the number of proteins encodable from a small genome. Examples: Adenoviruses, SV40, HIV (uses both spliced and unspliced mRNAs).

What is suppression of termination as a viral genome strategy?

Suppression of termination (readthrough) occurs when the ribosome occasionally ignores a stop codon and continues translating, producing a longer fusion protein. A special suppressor tRNA or structural element in the mRNA promotes readthrough at low frequency. This allows one mRNA to produce two proteins: a shorter version (from normal termination) and a longer version (from readthrough). Example: Retroviruses use readthrough to produce the Gag-Pol fusion protein.

What is translational reinitiation as a viral genome strategy?

Reinitiation occurs when a ribosome that has just finished translating a short upstream ORF (uORF) does not fully dissociate from the mRNA but instead "reinitiates" translation at a downstream AUG. This allows one mRNA to encode multiple proteins from sequential ORFs. Used by some viruses (and also by some cellular mRNAs) to expand the protein-coding potential of a single transcript.

Why do viruses suppress host gene expression, and what are the general strategies used?

WHY: By shutting down host gene expression, viruses divert ALL of the cell's ribosomes, energy, and raw materials to viral protein synthesis. They also prevent the host from producing antiviral factors (interferons, immune effectors) that would otherwise limit viral replication. HOW: Viruses can degrade host mRNAs (e.g., T4 nuclease degrades E. coli chromosome), block host RNA polymerase (modify sigma factors), inhibit cap-dependent translation (so only IRES-containing viral mRNAs are translated), or block nuclear export of host mRNAs while viral mRNAs are exported normally.

What are the seven key strategies viruses use to compress genetic information into small genomes?

1) Overlapping ORFs (same nucleotides read in different frames or from both strands). 2) mRNA splicing (one pre-mRNA → multiple mRNAs). 3) Polycistronic/multicistronic mRNAs (one mRNA → multiple proteins). 4) Ribosomal frameshifting (shift reading frame mid-translation). 5) Translational reinitiation (sequential ORFs on one mRNA). 6) Suppression of termination/readthrough (occasional readthrough of stop codon). 7) Proteolytic processing (one polyprotein cleaved into many functional proteins). 8) IRES (cap-independent internal translation initiation).

Summarize the key take-home messages of Baltimore Classification in four points.

1) ALL viruses must replicate their genomes and transcribe genes to produce viral proteins using CELLULAR ribosomes. 2) DNA viruses generally replicate in the NUCLEUS and may use cellular polymerases (Poxviruses are the exception — cytoplasmic). 3) RNA viruses replicate in the CYTOPLASM but must encode and carry their own polymerases — tend to have smaller, more error-prone genomes. 4) Viruses are classified by the nature of their genome (DNA/RNA, ss/ds, +/−) and the pathway to mRNA production. They have evolved extraordinary strategies to compress maximum information into minimum genome size.

What is the one universal rule that applies to ALL seven Baltimore classes without exception?

All viruses, in all seven classes, must ultimately produce mRNA in the POSITIVE (+) sense that can be translated by HOST CELL RIBOSOMES. No virus can translate its own proteins — ribosomes are the one component that every virus absolutely depends on the host to supply. This is the unifying principle of the Baltimore Classification.

What did the original Tobacco Mosaic Virus (TMV) filtration experiments by Ivanovsky and Beijerinck demonstrate?

Crushed TMV-infected leaves were filtered through a porcelain filter that removed bacteria. The filtrate (cell-free liquid) could still infect a new tobacco leaf — proving the infectious agent was smaller than bacteria. Critically, the agent could NOT grow on its own without a host — this was the fundamental distinction from bacteria, which do not require a host to replicate.

What were the two key conclusions that distinguished the TMV agent from bacteria?

1) The infectious agent passed through filters that retained bacteria — meaning it is sub-bacterial in size. 2) The agent could not "grow" or replicate on its own — it required a living host cell. These two observations established that viruses were a fundamentally new class of infectious agent, not simply very small bacteria.

What is the Adsorption period on a viral growth curve?

The adsorption period is the time immediately after virus is added to cells during which the virus particles attach to the host cell surface (adsorption = attachment). During this phase, free extracellular virus disappears from the medium as virions bind to host cell receptors.

What is the Eclipse period on a viral growth curve and what causes it?

The eclipse is a period when infectivity (detectable virus) completely disappears from the system. It is caused by UNCOATING — the virus has entered the cell and disassembled, releasing its genome. The individual viral components (nucleic acid, protein) are not yet infectious on their own, so no infectious units are detectable. It precedes the latent period.

What is the Latent period on a viral growth curve?

The latent period is the time following eclipse during which the virus is replicating its genome and synthesizing viral proteins (early enzymes, nucleic acid, protein coats) using host cell machinery. No new virions have been assembled yet. Ends when maturation begins.

What is the Maturation phase on a viral growth curve?

Maturation is the assembly of newly synthesized viral genome copies and viral proteins into complete, infectious virion particles. Represents the final intracellular stage before release. In animal viruses this phase occurs 8-40 hours post-infection.

What happens during the Assembly and Release phase of the viral growth curve?

Fully assembled virions are released from the host cell — either by cell lysis (non-enveloped viruses), budding (enveloped viruses), or exocytosis. The extracellular virus count rises sharply. This is reflected as the "rise" on the one-step growth curve.

How does viral replication fundamentally differ from bacterial replication?

Bacteria grow and divide as whole cells (binary fission) — they always exist as intact organisms. Viruses do NOT grow as intact particles. Instead, they disassemble upon entry (eclipse), make their bits and pieces separately using host cell machinery, and then REASSEMBLE into new infectious virions. A virus is never "half a virus" growing larger.

What is the one-step growth curve and what does it tell you?

The one-step growth curve is an experimental plot of extracellular infectious virus (plaque-forming units/mL) over time following a synchronized, single-round infection. It reveals: the eclipse period (no infectivity), the latent period (intracellular replication), the rise period (burst of released virions), and the burst size (total virions released per infected cell).

What are the three types of host systems used to culture viruses in the laboratory?

1) Whole animal hosts: e.g., primates and mice — most physiologically relevant but expensive and ethically complex. 2) Embryonated chicken eggs: fertilized eggs containing multiple tissue/cell types (chorioallantoic membrane, amnion, yolk sac, embryo) — cheaper, still used for influenza vaccine production. 3) Cell/tissue culture: cells grown in flasks with nutrient media — most common, cheapest, and most controllable.

Why are fertilized chicken eggs used to grow influenza virus, and what are the injection sites?

Fertilized chicken eggs are composed of multiple cell types that support influenza replication. They are cheaper and less ethically complex than whole animal models. Injection sites: chorioallantois, chorioallantoic membrane, amnion, yolk sac, or directly into the embryo. Still the primary method used to produce seasonal influenza vaccines at large scale.

What are Cytopathic Effects (CPEs)?

Cytopathic effects (CPEs) are the visible changes that a virus induces inside infected cells, observable by light microscopy. They are the hallmarks of viral infection in cell culture and can include: cell lysis, syncytia formation, and cellular transformation. Different viruses produce characteristic CPEs that can aid in identification.

What is cell lysis as a CPE?

Cell lysis is the destruction and rupture of the infected host cell, caused by the accumulation of viral particles and/or viral proteins that disrupt membrane integrity. Common in non-enveloped lytic viruses (e.g., Adenovirus, Poliovirus). Under microscopy, cells detach from the culture surface, round up, and eventually disappear, leaving clear areas.

What are syncytia and which virus is a classic example?

Syncytia (singular: syncytium) are large, multinucleated cell masses formed when viral envelope glycoproteins on the surface of an infected cell fuse the plasma membranes of adjacent uninfected cells together. Multiple nuclei become enclosed in one large cytoplasmic mass. Classic example: HIV induces syncytium formation. Also seen with Paramyxoviruses (measles, RSV).

What is cellular transformation as a CPE and what does it look like microscopically?

Viral transformation (an oncogenic CPE) occurs when a virus converts a normal cell into a cancer-like cell. Transformed cells lose contact inhibition — they are no longer flat but divide uncontrollably, piling up into foci of dense, rounded cells on top of each other. This is the basis of the focus formation assay used in cancer biology to detect oncogenic viruses.

What is a Plaque Assay and what was it first developed for?

The plaque assay is the primary method for measuring the infectious titer (concentration) of a virus. It was first developed for bacteriophages (viruses that infect bacteria). Principle: virus is mixed with a lawn of susceptible cells/bacteria, infected cells lyse and create clear circular zones (plaques) visible to the naked eye. Each plaque represents one infectious particle.

What is a plaque?

A plaque is a clear circular zone of cell death/lysis visible in an otherwise opaque lawn of cells or bacteria on an agar plate. Each plaque originates from a single infectious virion that infected one cell, replicated, released progeny that infected neighbouring cells, and so on — creating an expanding zone of destruction limited by the agar overlay.

Why is an agar overlay used in the plaque assay?

After the virus is added to cells, a gel-like agar overlay is added on top. The viscous agar RESTRICTS the diffusion of newly released progeny virions so that they can only infect immediately adjacent cells. This prevents virions from spreading freely through liquid, ensuring each plaque stays localized and countable as a discrete, individual zone.

What is a Plaque Forming Unit (PFU) and what does it measure?

A PFU (plaque-forming unit) is the unit of viral infectivity measured by the plaque assay. One PFU = one infectious virion (capable of producing a plaque). The viral titer is expressed as PFU/mL of the original stock and represents the INFECTIOUS particle count — NOT the total particle count.

How is PFU/mL of an original virus stock calculated from a plaque assay?

Serial dilutions (usually 10-fold) are made of the virus stock. 1 mL of each dilution is added to cells. After incubation, plaques are counted on plates with countable numbers. Formula: PFU/mL of original stock = (average number of plaques) × (dilution factor). Example: 12 plaques at 10⁸ dilution = 12 × 10⁸ = 1.2 × 10⁹ PFU/mL.

What is a 10-fold serial dilution and why is it used in plaque assays?

1 mL of virus is added to 9 mL buffer = 1:10 (10⁻¹) dilution. This is repeated 8 times, giving dilutions of 10¹ to 10⁸. Serial dilution is used because: (1) the original stock may have too many virions to count individual plaques; (2) you need countable plates (typically 30-300 plaques per plate); (3) multiplying plaque count by dilution factor back-calculates the original concentration.

Why is viral infectious titer (PFU/mL) NOT the same as total viral particle count?

Not every viral particle in a stock is actually infectious. Many particles may be: physically damaged, incompletely assembled, lacking a genome, or have misfolded surface proteins. The ratio of total particles to infectious particles varies greatly between viruses — some highly pure stocks have ratios of ~1:1, while others may have 10, 100, or even 1000 total particles per 1 infectious unit (PFU).

What is the Focus Formation Assay and when is it used?

The focus formation assay is used for oncogenic (cancer-causing) viruses that TRANSFORM cells rather than lyse them. Principle: virus is added to a monolayer of normal cells and covered with agar. Instead of plaques (clear zones), foci of transformation (dense piles of rounded, uncontrollably dividing cells) are detected visually or with staining. Each focus originated from one transforming viral particle. Crucial for studying oncogenic viruses.

What is the key difference between a plaque and a focus of transformation?

Plaque: clear/transparent zone in a cell monolayer caused by cell LYSIS (death) — used for lytic viruses. Focus of transformation: dense, opaque pile of rounded, abnormal cells — caused by uncontrolled PROLIFERATION (not death) driven by oncogenic virus transformation. Plaques = cell destruction; foci = uncontrolled cell growth. Both originate from a single infectious particle.

Why are plaque assay and focus formation assay described as "crucial for virology and cancer biology"?

Plaque assay: allows precise quantification of infectious virus (PFU/mL), essential for any experiment requiring defined doses of virus. Also reveals lytic behavior and CPE type. Focus formation assay: allows quantification of transforming/oncogenic viral particles and study of how oncogenic viruses convert normal cells to cancerous phenotypes. Together they are the foundational quantitative tools of experimental virology.

What is the take-home message of Lecture 3 on virology techniques?

1) Not all viruses are created equal — different viruses require different assays and host systems. 2) Different assays are used for different viruses (plaque for lytic; focus formation for transforming; CPE observation for diagnosis). 3) Viral infectious titer (PFU/mL) ≠ total number of viral particles — the ratio differs greatly between viruses. 4) Plaque assay and focus formation assay are the two most crucial quantitative techniques in virology and cancer biology.

What are the four key terms from the viral growth curve and their definitions in sequence?

1) Adsorption = attachment of virus to host cell surface. 2) Eclipse = period where infectivity disappears due to uncoating — genome is free but not yet replicated. 3) Latent = genome replication and viral protein synthesis underway — no assembled virions yet. 4) Maturation = assembly of genome + proteins into complete virions, followed by assembly and release.

What is the difference between the eclipse period and the latent period?

Eclipse: the sub-period within the latent phase immediately after uncoating, during which infectivity is ZERO — neither intact input virions nor new virions are detectable because the genome is free but not yet replicated. Latent: the broader period encompassing eclipse PLUS the time during which nucleic acid, early enzymes, and protein coats are being synthesized — ends when the first mature virions are assembled. Eclipse is contained within the latent period.

How ubiquitous are viruses in our everyday environment?

Viruses are present in the air we breathe, the food we eat, and everything we touch. Every milliliter of seawater contains more than one million virus particles. We eat and breathe billions of viruses daily. Viral infections constantly cross species barriers (zoonotic infections). The vast majority have little or no impact on our health because our immune system evolved to fight them.

What percentage of human DNA is made up of ancient viral genetic material?

Approximately 8% of human DNA consists of ancient retroviral genetic material. Every human cell contains thousands of copies of old and new retrovirus genomes integrated into our DNA — this material is passed from parent to child through the germline across generations.

What role do endogenous retroviruses play in human development?

During embryonic implantation in most mammals, endogenous retroviruses (ERVs) are activated and expressed. They are thought to play a role in gestational immune tolerance and in the formation of the placental syncytium — the multi-nucleated layer of cells at the fetal-maternal interface. This suggests ancient viral sequences have been co-opted for essential biological functions.

What are the three essential strategies shared by ALL viruses for survival?

1) All viruses package their genomes inside a particle (virion) used for transmission of the genome from host to host. 2) The viral genome contains all information needed to initiate and complete an infectious cycle inside a susceptible and permissive host cell. 3) All viral genomes can establish themselves in a host population to ensure long-term viral survival.

What are the two universal facts that underlie the simplicity of all viruses?

1) All viral genomes are obligate molecular parasites — they can only function after infecting a host cell. 2) All viruses must make mRNA that can be translated by HOST CELL RIBOSOMES — they are all parasites of the host protein synthesis machinery. No virus is large enough to encode all the apparatus needed for protein synthesis.

What does "a viral infection is an exercise in cell biology" mean?

Viruses are too small to encode everything needed for replication. They completely depend on host cell functions including: ribosomes for translation of viral mRNAs, cellular energy (ATP), enzymes for replication and assembly, and cellular transport pathways to move viral components around the cell. Studying how viruses exploit these pathways reveals fundamental cell biology.