July 30th lecture

Third Major Virus Type: Bacteriophages ("Phages")

• Infect bacteria exclusively; absent in eukaryotes.

• Name origin

  • “Phage” = “eat.” Early observers saw “holes” (plaques) in a lawn of growing bacteria, assumed something was eating them.

• Plaque dynamics: virus infects → lyses host → progeny spread radially producing expanding clear zones.

• T4 phage (classic example)

  • Infects Escherichia coli.

  • Morphology:

  • Icosahedral “head” (payload of nucleic acid).

  • Contractile “tail sheath.”

  • Tail fibers (long, spider-leg-like) for host recognition and attachment.

  • Must penetrate peptidoglycan—unique challenge vs. eukaryotic viruses.

Universally Required Viral Components (All Viruses)

• Genome (Instructions) – four mutually exclusive possibilities:

  1. Single-stranded DNA (ssDNA) ★

  2. Single-stranded RNA (ssRNA) ★

  3. Double-stranded DNA (dsDNA)

  4. Double-stranded RNA (dsRNA) ★
     ★ = never serves as genome in living cells, underscoring viral non-living status.

• Capsid – hard protective shell.

  • Built from repeating globular capsomeres (protein subunits, often named VP – viral protein – with size number, e.g., VP5, VP26).

• Nucleocapsid = Genome + Capsid considered together (minimal infectious unit).

• Internal “fluid” is not true cytoplasm—no cytoskeleton, no metabolic reactions; stolen remnant from prior host.

Capsid Geometry & Terminology

• Icosahedral symmetry (very common)

  • Geometric solid of 20 triangular faces; resembles a D20 die.

  • Viewed in 2-D, often looks octagonal.

• Helical/Filamentous capsids

  • Capsomeres bind directly along length of nucleic acid forming a coiled rod/filament (e.g., influenza, Ebola, rabies).

• Illustrative examples

  • Herpes Simplex Virus-1 nucleocapsid comprises VP5 pentons, VP26 & VP hexons; surface folds create specific ligand shapes for cell-receptor binding.

Enveloped vs. Naked Viruses

Naked (Unenveloped)

• Composition: nucleocapsid only (hence described as “naked”).

• Surface capsomeres themselves serve as ligand/attachment proteins.

• Environmental robustness: tolerate drying, acidity, temperature swings.

• Transmission can be indirect (fomites, food, water).

• Immune visibility: surface is entirely “foreign” → easier target once inside host.

Enveloped

• Composition: nucleocapsid plus outer phospholipid bilayer stolen from last host membrane (plasma, ER, or nuclear envelope).

• Envelope is generally spherical, leaving a visible gap between lipid shell & non-spherical nucleocapsid.

• Peplomers (Spikes)

  • Glycoproteins inserted through envelope; essential for attachment/entry because capsomeres are internal.

  • Example: SARS-CoV-2 “spike protein.”

• Pros & Cons

  • Advantage inside host: envelope mimics “self,” helps evade immune detection.

  • Liability outside host: lipid bilayer fragile—requires warm, moist, isotonic conditions; detergents, desiccation, acid destroy it, simultaneously removing peplomers → loss of infectivity.

• Practical outcome: enveloped viruses typically spread by direct transmission (respiratory droplets, blood, sexual contact, direct touch) rather than contaminated surfaces/food/water.

• Key observations

  • Hepatitis viruses (A, B, C) share target organ (liver) but fall into three unrelated families with different genome types & envelope status ➔ clinical similarities arise from host-cell choice, not genetic relatedness.

  • Double-stranded RNA (Reoviridae) is unique to viruses—no cellular life uses dsRNA genome.

Drug Targets & Life-Cycle Relevance (Preview)

• Students are not responsible for memorizing antiviral drug names, but must relate a described mechanism to:

  • Which stage of viral life cycle it inhibits.

  • Which life-cycle variant (lytic vs. lysogenic) it pertains to.

  • Example: “Drug X blocks reverse transcriptase” → affects synthesis stage in retroviral replication.

Numerical & Structural Facts (Quick Reference)

• Human genome: 46 chromosomes (dsDNA); bacterial genome: 1 circular dsDNA; viral genomes: 4 structural categories (ssDNA, dsDNA, ssRNA, dsRNA).

• Icosahedral capsid: 20 equilateral-triangle faces.

• Influenza: 8 unique ssRNA segments, each packaged individually within one envelope.

Take-Home Messages

• Minimal viral architecture = nucleocapsid; everything else (envelope, peplomers, tails) is optional or family-specific.

• Envelope status dictates environmental durability and transmission mode.

• Genome type drives replication strategy diversity → core reason synthesis stage is complex and drawing practice is required.

• Upcoming assessments (lab test, drawings, bonus chart) purposely scaffold high-difficulty content—begin review early.