Viruses

Viruses

Part 1 of 4 — Viral Architecture & the Infectious Cycle

What a Virus Is (and Is Not)

A virion is genetic material in a protein box: a DNA or RNA genome (never both) inside a capsid built from repeating protein subunits (capsomeres), sometimes wrapped in a host-derived lipid envelope studded with viral glycoproteins. Viruses are obligate intracellular parasites: no ribosomes, no ATP-generating metabolism, no ability to replicate outside a cell. They fail most definitions of life — no growth, no homeostasis, no independent reproduction — yet they evolve by mutation and selection. Size scale: ~20-300 nm, generally below light-microscope resolution and small enough to pass bacteria-retaining filters (their original discovery signature).

Genome Diversity — the Widest in Biology

Viral genomes may be DNA or RNA; single- or double-stranded; linear, circular, or segmented (in pieces); and as small as a few genes. Two housekeeping conventions:

• Positive (+) sense RNA = readable directly as mRNA.
• Negative (−) sense RNA = complement of mRNA; must be transcribed before translation (details in Part 3).

Enveloped vs naked predicts real behavior: envelopes (acquired by budding through host membranes) are damaged by detergents, drying, and stomach acid — enveloped viruses favor direct contact/respiratory transmission. Naked capsids are tougher: fecal-oral routes, surfaces, and GI survival belong to naked viruses.

The Six-Step Infectious Cycle

1. Attachment (adsorption): viral surface protein binds a specific host receptor — the interaction that defines tropism (which species and cell types are infectable). No receptor, no infection.
2. Penetration: receptor-mediated endocytosis, or (enveloped only) membrane fusion.
3. Uncoating: capsid disassembles, releasing the genome.
4. Synthesis: genome replication + mRNA production + protein translation — strategy depends entirely on genome type (Parts 2-3).
5. Assembly: capsids self-assemble around new genomes.
6. Release: lysis (kills the cell — typical of naked viruses) or budding through a membrane (acquires the envelope; the cell may survive for a while, continuously shedding virions).

Quantifying Infection

The plaque assay counts infectious units: virus dilutions on a cell monolayer produce clear zones (plaques), each founded by ONE infectious virion — reported as plaque-forming units (PFU). The one-step growth curve shows an eclipse period (after uncoating, NO intact virions exist inside the cell — infectivity vanishes before it multiplies) followed by a burst of progeny per cell. The eclipse is conceptual gold: a virus is information, not a dividing cell; it disassembles to reproduce.

Viral Structure & Life Cycle 🎯

Key Takeaways — Part 1

• Virion = DNA or RNA (one type only) + capsid, with or without a host-derived envelope; no ribosomes, no metabolism — obligate intracellular parasite
• Genome menu: DNA/RNA, ss/ds, linear/circular/segmented; + sense reads as mRNA, − sense must be transcribed first
• Enveloped = fragile (contact/droplet spread); naked = rugged (fecal-oral, surfaces)
• Cycle: attach (receptor = tropism) → penetrate → uncoat → synthesize → assemble → release (lysis vs budding)
• Plaque assay counts infectious units (PFU); eclipse period = zero infectivity between uncoating and first assembly

Worked Examples — Life-Cycle Reasoning

Question: A synchronized infection shows: intracellular infectivity drops to zero at t = 0-8 h, rises steeply at 8-16 h, and extracellular virions appear only after 16 h. Assign the phases and predict what electron microscopy would show at 6 h.

Solution:

1. 0-8 h = eclipse: genomes and proteins are being synthesized; EM at 6 h shows NO intact virions — only replication complexes and free components.
2. 8-16 h = assembly: intracellular infectious particles accumulate.
3. After 16 h = release (here delayed and abrupt — consistent with lysis rather than continuous budding).

MCAT Strategy: Match curve features to cycle steps; 'infectivity disappears then reappears' is the eclipse signature unique to viral reproduction.

Question: Drug 1 is a soluble decoy mimicking the host receptor; drug 2 blocks endosomal acidification; drug 3 inhibits the viral protease that trims capsid proteins. For each, state the step blocked and whether already-infected cells are protected.

Solution:

1. Drug 1 binds virion attachment proteins → blocks ATTACHMENT; protects uninfected cells only.
2. Many enveloped viruses fuse only after endosomal acidification triggers their fusion protein → blocks PENETRATION/uncoating; again prophylactic, useless post-entry.
3. Drug 3 blocks maturation during ASSEMBLY → acts INSIDE infected cells, preventing production of infectious progeny (the logic of HIV protease inhibitors).
4. Rule: entry-step drugs prevent; late-step drugs suppress ongoing production.

MCAT Strategy: For any antiviral, first place it on the six-step cycle; the placement answers 'who is protected' automatically.

Question: Influenza virions agglutinate red blood cells by binding sialic acid. Patient serum taken after vaccination blocks this agglutination. What does the assay measure, and why does it correlate with protection?

Solution:

1. Each virion carries many hemagglutinin (HA) spikes; by binding sialic acid on multiple RBCs simultaneously, virions crosslink cells into a lattice.
2. Anti-HA antibodies coat the receptor-binding site, so no crosslinking — the hemagglutination-inhibition titer counts receptor-blocking antibody.
3. The same HA-sialic acid interaction is the virus's ATTACHMENT step, so antibodies that inhibit agglutination also neutralize infection.

MCAT Strategy: Assays often exploit the same molecular event as pathogenesis; identify the shared interaction and the correlation explains itself.

Viruses

Part 2 of 4 — Bacteriophages: Lytic & Lysogenic Cycles

The Phage Body Plan

Bacteriophages — viruses of bacteria — include the iconic tailed phages (T4, lambda): an icosahedral head packed with dsDNA, a contractile or non-contractile tail, and tail fibers that recognize receptors on the bacterial surface (LPS, pili, teichoic acids). Injection is distinctive: the tail contracts like a syringe and drives the DNA through the envelope — the capsid stays outside. Hershey and Chase exploited exactly this: $^{32}$P-labeled DNA entered cells, $^{35}$S-labeled protein did not, proving DNA is the genetic material.

The Lytic Cycle (Virulent Phages, e.g., T4)

Injection → immediate takeover: host transcription is redirected, early genes (nucleases, replication enzymes) degrade host DNA and copy the phage genome; late genes build capsids and lysis proteins. Assembly, then lysozyme-mediated wall rupture releases a burst of progeny. Virulent phages have exactly one setting: kill.

• For T4, the burst is ~100-200 progeny per cell, about 25 minutes after infection.

The Lysogenic Cycle (Temperate Phages, e.g., Lambda)

A temperate phage has a choice. Instead of replicating, its genome can integrate into the host chromosome (site-specific recombination by phage integrase) as a prophage:

• The prophage is replicated PASSIVELY with the chromosome — every daughter cell inherits it. The carrier cell is a lysogen.
• A phage-encoded repressor protein silences the lytic genes AND confers superinfection immunity: another phage of the same type injecting its DNA finds the repressor already waiting.
• Induction: DNA damage (UV, mutagens) leads to destruction of the repressor → the prophage excises and enters the lytic cycle. Teleologically: the ship is sinking, launch the lifeboats.

• The damage sensor is the host SOS response: activated RecA promotes self-cleavage of the phage repressor.

• The lysis-lysogeny decision itself tilts with conditions: high multiplicity of infection and starved hosts favor lysogeny (poor prospects for a burst); rich, sparse conditions favor lysis.

Why Lysogeny Matters Medically

Lysogenic conversion: prophage genes change the host's phenotype. Diphtheria toxin, cholera toxin, and the Shiga toxins of enterohemorrhagic E. coli are all encoded by prophages (as are some botulinum toxin types) — the bacterium is only pathogenic while 'infected.' And specialized transduction is a lysogeny error: an imprecisely excising prophage carries flanking host genes to its next host (vs generalized transduction, a lytic packaging error that can move any gene — see the Bacteria lesson).

Phages as Tools

Plaque assays, transduction-based gene mapping, phage display, and modern phage therapy against multidrug-resistant bacteria all rest on this biology. CRISPR-Cas immunity is the bacterial counterattack: captured phage sequence fragments (spacers) guide Cas nucleases to destroy re-invading phage DNA — an adaptive immune system in a single cell.

Phage Cycles & Lysogeny 🎯

Key Takeaways — Part 2

• Tailed phage = DNA-packed head + syringe tail; DNA injected, capsid stays out (Hershey-Chase: 32P in, 35S out)
• Lytic (virulent, T4): early genes hijack and copy, late genes build and lyse — burst of progeny
• Lysogenic (temperate, lambda): integrase inserts prophage; repressor silences lytic genes + grants superinfection immunity
• Induction: DNA damage → repressor destroyed → excision → lysis; starvation + high phage density favor lysogeny instead
• Lysogenic conversion arms bacteria with phage-encoded toxins (diphtheria, cholera, Shiga)
• Sloppy prophage excision = specialized transduction; lytic mispackaging = generalized; CRISPR = bacterial adaptive defense

• Low-yield extras: T4 bursts ~100-200 progeny in ~25 minutes; the SOS sensor RecA triggers repressor self-cleavage during induction; the lambda repressor gene is cI (clear-plaque mutants are cI-minus)

Worked Examples — Phage Genetics

Question: Wild-type lambda forms turbid (cloudy) plaques; a mutant forms perfectly clear plaques. Propose the mutation and explain the plaque morphologies.

Solution:

1. Within a wild-type plaque, some infected cells choose LYSOGENY, survive (immune to superinfection), and keep growing — a haze of live lysogens clouds the cleared zone: turbid.
2. Clear plaques mean NO survivors — the mutant cannot establish lysogeny. The classic lesion is a defective repressor gene.
3. Corollary: a repressor-defective phage can still grow lytically (plaques form) but can never form stable lysogens.

MCAT Strategy: Plaque morphology is a phenotype readout of the lysis-lysogeny circuit; turbidity = lysogens = functional repressor.

• The lambda repressor gene is named cI; clear-plaque mutants are cI-minus.

Question: Lambda integrates between the E. coli gal and bio genes. A rare transducing lysate moves gal+ or bio+ into recipients, but never trp+ (located far away). Explain, and contrast with phage P1 generalized transduction.

Solution:

1. Specialized transduction requires IMPRECISE prophage excision: the loop-out accidentally captures DNA adjacent to the attachment site — only gal or bio can be included; trp is never adjacent.
2. The resulting particle is usually defective (it left phage genes behind in trade).
3. P1 (generalized) mispackages random chromosome fragments during LYTIC growth: any locus, including trp, moves at roughly comparable low frequency.

MCAT Strategy: Ask where the phage DNA sits when the error happens: integrated (specialized → neighbors only) vs never integrated (generalized → anything).

Question: A patient's multidrug-resistant Pseudomonas isolate is tested against a phage library. Phage 12 lyses it today; a week into therapy, resistant bacterial colonies emerge whose surfaces lack the type IV pilus. Explain the resistance and one countermeasure.

Solution:

1. Phage 12 evidently uses the type IV pilus as its receptor; losing the pilus removes the attachment site — resistance by receptor loss (the phage version of 'keep the drug out').
2. Countermeasure: a phage COCKTAIL targeting several distinct receptors — simultaneous loss of all is improbable; bonus if receptor loss costs the bacterium virulence (pili are adhesins).
3. Note the evolutionary trade: phage resistance here made the pathogen less able to adhere to host tissue.

MCAT Strategy: Phage-host coevolution mirrors antibiotic resistance logic — receptor alteration, restriction, CRISPR — and cocktails mirror combination therapy.

Viruses

Part 3 of 4 — Animal Virus Replication Strategies

One Question Organizes Everything: How Do You Make mRNA?

Host ribosomes only translate mRNA. Every genome type is a different distance from mRNA, and that distance dictates what the virion must CARRY versus what it can borrow:

Two corollaries worth deriving, not memorizing: purified (+)RNA genomes alone can launch an infection (ribosomes accept them directly); purified (−)RNA or dsRNA genomes CANNOT (the required RdRp was in the discarded virion). And host cells have NO RNA-dependent RNA polymerase — virtually every RNA virus except retroviruses must encode one.

The Retrovirus Exception to the Central Dogma

HIV carries two (+)RNA copies plus reverse transcriptase (RT): RNA → dsDNA, which integrase inserts into the host genome as a provirus — permanent, replicated with every cell division (the molecular reason HIV cannot be cured by suppressing replication alone; compare lysogeny). Host RNA polymerase II then transcribes progeny genomes and mRNA; protease matures the budding particle. Each enzyme is a drug target: RT inhibitors (nucleoside chain terminators like AZT), integrase inhibitors, protease inhibitors. RT is error-prone with no proofreading → HIV's ferocious mutation rate → combination therapy (HAART) is mandatory: a single drug fails because resistant mutants already exist in the viral population before treatment begins.

• That pre-existing-mutant argument is the Luria-Delbruck logic: resistance mutations arise at random, and the drug only selects them.

RNA Virus Sloppiness & Its Consequences

Most viral RdRps also lack proofreading (coronaviruses are a rare exception): mutation rates thousands of times or more above DNA-based genomes. Consequences: quasispecies swarms and rapid antigenic drift (point mutations in surface proteins — why flu vaccines update annually).

• A third consequence: strict genome-size ceilings — past them, mutations accumulate faster than selection can purge them (error catastrophe).

Segmented genomes (influenza's 8 RNA pieces) add reassortment: two strains co-infecting one cell shuffle whole segments — antigenic shift, the pandemic mechanism (covered further in Part 4).

Envelope Acquisition & Latency

Enveloped animal viruses bud through a host membrane pre-studded with viral glycoproteins — plasma membrane (HIV, flu) or internal membranes (herpesviruses first bud through the nuclear envelope). Some DNA viruses establish latency without integration: herpesviruses persist as circular episomes in neurons, reactivating under stress — same strategic idea as lysogeny, different molecular bookkeeping (episome vs provirus vs prophage).

Replication Strategies 🎯

Key Takeaways — Part 3

• Organize by the route to mRNA: dsDNA borrows host pol (nucleus); (+)RNA IS mRNA (naked genome infectious); (−)RNA and dsRNA must PACK RdRp; retroviruses pack RT
• Host cells have no RdRp — RNA viruses bring or immediately build their own
• HIV: RT → dsDNA → integrase → provirus (permanent reservoir) → host RNA pol II; RT, integrase, protease = the drug targets
• No proofreading in RdRp/RT → quasispecies, antigenic drift, mandatory combination therapy
• Segmented genomes reassort in co-infected cells → antigenic shift (pandemics)
• Persistence bookkeeping: prophage (phage, integrated), provirus (HIV, integrated), episome (herpes, not integrated)

• Low-yield extras: the pre-existing-resistant-mutant argument is the Luria-Delbruck logic; error catastrophe caps RNA genome size; actinomycin D is the classic DNA-templated transcription blocker used to rule out DNA steps

Worked Examples — Strategy Deduction

Question: Virus X: (a) replication is unaffected by a drug that blocks DNA-templated transcription; (b) its purified genome is NOT infectious; (c) virions contain a polymerase. Classify the genome.

Solution:

1. (a) No DNA step anywhere → rules out DNA viruses AND retroviruses (whose proviral transcription needs DNA templates).
2. (b) Not infectious alone → not (+) ssRNA.
3. (c) Packaged polymerase confirms it: (−) ssRNA or dsRNA — an RdRp-carrying RNA virus.
4. To split the final pair: check whether the genome is RNase-resistant when double-stranded (dsRNA) or sensitive (ss).

MCAT Strategy: The three classic probes — sensitivity to a DNA-templated transcription blocker, naked-genome infectivity, virion polymerase — uniquely bin every genome class. Practice the flowchart.

• The classic DNA-templated transcription inhibitor used in this test is actinomycin D.

Question: Two influenza strains co-infect one cell: strain A contributes an HA the patient has antibodies against; strain B's HA is novel. Some progeny permanently encode B's HA with A's other seven segments. Is this drift, shift, or phenotypic mixing?

Solution:

1. Whole SEGMENTS were exchanged and the change is heritable → reassortment = antigenic shift. A hybrid genotype now carries a surface protein the population has never seen: pandemic potential.
2. Drift would be gradual point mutations in HA, not a wholesale swap.
3. Phenotypic mixing is the transient case: a progeny genome wrapped in the OTHER strain's proteins without genetic change — one generation later, the coat matches the genes again.

MCAT Strategy: Ask two questions: is the change heritable (genome) or cosmetic (coat)? Gradual (drift) or wholesale (shift)? Segmented genomes are the prerequisite for shift.

Question: Arrange where these act in a single round of HIV infection: protease inhibitor, CCR5 antagonist, integrase inhibitor, nucleoside RT inhibitor — and state which steps are irreversible for that cell.

Solution:

1. CCR5 antagonist — blocks co-receptor binding at ENTRY (step 1-2).
2. NRTI — chain-terminates reverse transcription in the cytoplasm (step 4a).
3. Integrase inhibitor — prevents proviral insertion (step 4b). Integration is the point of no return: after it, the cell is a permanent reservoir.
4. Protease inhibitor — blocks maturation of budding particles (step 5-6); the cell still produces virions, but they are non-infectious.

MCAT Strategy: Anchor each drug to the numbered life-cycle step; 'reversibility' questions are really asking whether integration has occurred yet.

Viruses

Part 4 of 4 — Viral Evolution, Subviral Agents & MCAT Integration

Drift, Shift, and Vaccine Logic

• Antigenic drift: the steady accumulation of point mutations in surface antigens (sloppy RdRp, Part 3). Antibodies from last year bind a little worse each season → annual influenza vaccine reformulation; epidemics.
• Antigenic shift: abrupt appearance of a novel surface protein via reassortment of segmented genomes in a co-infected host (classically pig cells co-infected with human and avian influenza — the 'mixing vessel'). No pre-existing population immunity → pandemics. Shift requires segments; non-segmented viruses can only drift and recombine.
• Vaccine platforms map onto virology: live attenuated (replicates weakly; strong, broad immunity including mucosal; risky in immunocompromised patients), inactivated/killed (safe, weaker, boosters needed), subunit (one purified antigen — choose the protein whose neutralization blocks attachment), and mRNA (host cells translate the delivered (+)-sense message into antigen — the same trick a (+)RNA virus plays on the ribosome).

Smaller Than Viruses

• Viroid specifics: genomes of only ~250-400 nucleotides; the classic example is potato spindle tuber viroid.

Prions violate every nucleic-acid expectation: no genome to mutate, no UV or nuclease sensitivity, extreme resistance to heat and standard autoclaving, no immune response (PrP is a self protein), and inheritance of SHAPE rather than sequence. Protease resistance of the beta-sheet aggregate is the standard lab signature.

Host Defenses & Viral Countermoves (Rapid Tour)

Infected cells secrete interferons, warning neighbors to degrade RNA and slow translation; cytotoxic T cells kill cells displaying viral peptides on MHC I; antibodies neutralize free virions. Viruses answer with latency (Part 3), antigenic variation (above), and MHC downregulation — which exposes them to natural killer cells (the missing-self backup). This arms race is a standing MCAT crossover with immunology.

The Capstone Habit

Any virus question yields to four sorted facts: (1) genome class → route to mRNA → what the virion must carry; (2) enveloped or naked → transmission and stability; (3) persistence mode → prophage/provirus/episome → curability; (4) mutation engine → proofreading-free polymerase, segments or not → drift/shift/therapy strategy. Practice stating all four for HIV, influenza, herpes, and polio — those four organisms span the entire logical space.

Evolution, Prions & Integration 🎯

Key Takeaways — Part 4

• Drift = gradual point mutations (epidemics, annual vaccines); shift = segment reassortment, novel antigen (pandemics); shift needs a segmented genome
• Vaccines: live attenuated (strong, replicates — avoid in immunocompromised), killed (safe, weaker), subunit (target the attachment protein), mRNA (deliver the message)
• Viroid = naked noncoding circular RNA (plants); prion = protein-only conformational replicator (UV/nuclease-resistant, protease-resistant PrP-Sc); HDV = satellite needing HBV's HBsAg
• Defense stack: interferon (warn), CTL via MHC I (kill infected), antibody (neutralize free), NK cells (missing self backup when viruses hide MHC I)
• Capstone sort for any virus: genome → mRNA route; envelope → transmission; persistence mode → curability; polymerase fidelity + segments → evolution and therapy

• Low-yield extras: viroid genomes are only ~250-400 nucleotides, and potato spindle tuber viroid is the classic example

Worked Examples — Integration Passages

Question: Design the two key experiments that distinguish a prion from a slow conventional virus, and state the expected results for each hypothesis.

Solution:

1. Radiation/nuclease attack: UV and nucleases destroy nucleic acid function. Virus: infectivity falls sharply. Prion: infectivity unchanged (observed result for scrapie — an early anomaly).
2. Genetic specificity test: infectious material's properties should track the HOST's PrP gene, not an agent genome. Mice lacking PrP-C entirely (PrP knockout) are COMPLETELY resistant and cannot propagate the agent — no substrate, no conversion. A virus would still replicate in knockout mice.
3. Together: the information is conformational, templated onto a host-encoded protein.

MCAT Strategy: The knockout-resistance result is the single most decisive fact; expect passages to present it and ask what it implies.

Question: A (+)RNA respiratory virus enters cells when its spike protein binds a host receptor; a nucleocapsid protein is highly abundant and strongly antigenic but internal. Which protein belongs in a subunit vaccine, and why do anti-nucleocapsid antibodies fail to protect?

Solution:

1. Antibody protection against infection requires NEUTRALIZATION — physically blocking attachment or fusion. Only the SPIKE is exposed on the virion surface and mediates entry.
2. Anti-nucleocapsid antibodies bind a protein hidden inside the particle: they cannot prevent entry (they may still serve as an infection marker, and internal antigens matter for T-cell responses).
3. Choose the spike; ideally its receptor-binding domain, the region where binding antibodies most directly block the attachment step (Part 1's cycle logic).

MCAT Strategy: Map every proposed antigen onto the life-cycle step its antibody could physically interrupt; abundance and antigenicity without accessibility protect nothing.

Question: Apply the capstone sort (genome, envelope, persistence, evolution engine) to polio and HIV, and derive one clinical consequence from each fact.

Solution:

1. Polio: (+)ssRNA, naked, no persistence mechanism, drift-only. Consequences: genome alone is infectious (lab safety); fecal-oral spread and surface stability (sanitation matters); complete clearance possible (eradication is feasible); antigenically stable enough for a fixed vaccine.
2. HIV: (+)ssRNA retrovirus (RT + integrase), enveloped, provirus persistence, hypermutable RT. Consequences: unique enzymes = drug targets; fragile envelope = no casual-contact transmission; integrated reservoir = suppressible but not yet curable; quasispecies = combination therapy and no vaccine so far.
3. Same nucleic acid class, almost opposite clinical realities — the downstream facts, not the genome label, carry the medicine.

MCAT Strategy: This paired contrast is worth rehearsing verbatim; passages that seem novel are usually one of these four axes wearing a costume.