Micro F Comprehensive Study Guide on Viruses, Viroids, and Prions

Nature and Characteristics of Viruses

• Definition and Nature:

  • Viruses are acellular infectious particles or agents composed of a nucleic acid genome surrounded by a protective protein coat.

  • Unlike all cellular living organisms (which exclusively utilize double-stranded DNA genomes), viruses can possess either DNA or RNA as their genetic material.

  • Viruses are entirely devoid of cellular architecture, lacking cytoplasmic membranes, cytosol, and metabolic organelles.

  • Viruses cannot carry out autonomous metabolic functions, grow, respond to environmental stimuli, or reproduce independently; they are obligate intracellular parasites that must hijack the cellular machinery of an intact host cell.

  • Viruses are responsible for the vast majority of infectious diseases that afflict the industrialized world.

• Physical States of Viruses:

  • Extracellular State (Virion):

    • Consists of a nucleic acid core enclosed by a protein coat called a capsid.

    • In some viral species, the capsid is further surrounded by a phospholipid membranous layer known as an envelope.

    • The outermost layer (whether the protein capsid or the envelope) carries specific viral surface proteins that mediate host cell recognition and binding.

  • Intracellular State:

    • Once a virion penetrates a host cell, the capsid and envelope (if present) are completely removed through uncoating.

    • In this state, the virus exists solely as bare, replicating nucleic acid.

• Host Specificity and Tropism:

  • Most viruses infect only a specific type of host cell due to precise complementary interactions between viral surface proteins and specific receptor proteins on the host cell membrane, analogous to an enzyme-substrate lock-and-key interaction.

  • Narrow Tropism: Certain viruses infect only one specific cell type within a single host species (e.g., viruses restricted strictly to human liver hepatocytes).

  • Generalist Viruses: Some viruses infect diverse tissue types or cross broad species barriers.

    • SARS-CoV-2 (COVID-19) is a prime example of a generalist virus because its target, the $ACE2$ receptor, is expressed on numerous distinct cell types in humans as well as across many other mammalian species.

  • Susceptibility across Kingdoms: All cellular organisms—animals, plants, fungi, and bacteria—are susceptible to infection by specialized viruses.

• Viral Dimensions and Microscopy:

  • Viruses are submicroscopic and cannot be viewed using a conventional light microscope.

  • Size Comparison:

    • Substantially smaller than eukaryotic cells, such as human red blood cells, and prokaryotic cells, such as $Escherichia\,coli$.

    • Size varies widely within the viral world, ranging from relatively large viruses such as smallpox virus to very small animal viruses such as poliovirus.

• Biological Status (Are Viruses Alive?):

  • Viruses are classified as non-living entities because they fail to meet the four foundational biological criteria of life:

    • They cannot grow.

    • They cannot reproduce independently.

    • They cannot respond autonomously to environmental stimuli.

    • They lack metabolic capabilities.

  • Certain scientists point out virus-associated characteristics that resemble living systems, including their ability to invade and hijack host cellular mechanisms, the possession of their own genetic genomes, and their capacity to evolve over time through genetic mutations (evidenced by the real-time evolution of SARS-CoV-2 and annual antigenic variations in influenza requiring updated seasonal vaccines).

Viral Structure, Morphology, and Classification

• Capsid Structure:

  • The capsid is a protective proteinaceous shell enclosing the viral genome.

  • Composed of organized, repeating protein subunits termed capsomeres.

  • Capsid morphology is a principal criterion for viral classification:

    • Helical capsids.

    • Icosahedral capsids (displaying 20 identical triangular facets).

    • Complex geometric shapes.

• Viral Envelope:

  • An outer lipid bilayer present in enveloped viruses; absent in non-enveloped (naked) viruses.

  • Origin: Acquired directly from host cell membranes during viral maturation and release via budding.

  • Chemical Composition: Composed of a host-derived phospholipid bilayer intermixed with both host membrane proteins and virally encoded proteins.

  • Spikes:

    • Virally encoded glycoprotein projections protruding from the surface of the envelope.

    • Function as essential receptor-binding ligands required to attach to and enter host cells.

  • Immunological Advantage:

    • The incorporation of host-derived membrane proteins allows enveloped viruses to mimic host tissue, effectively concealing the virion from host immune surveillance.

  • Fragility and Environmental Sensitivity:

    • Enveloped viruses are paradoxically more fragile and less durable than naked viruses.

    • The lipid bilayer is susceptible to disruption by environmental temperatures, desiccation, detergents, and chemical agents.

    • If the envelope is compromised, the spike proteins required for host receptor binding are stripped away; because the internal capsid of an enveloped virus lacks host-attachment proteins, a stripped virion becomes completely non-infectious.

• Viral Genome Organization:

  • Can consist of DNA or RNA, existing as either double-stranded ($ds$) or single-stranded ($ss$) molecules:

    • Double-stranded DNA ($dsDNA$).

    • Single-stranded DNA ($ssDNA$).

    • Double-stranded RNA ($dsRNA$).

    • Single-stranded RNA ($ssRNA$).

  • Genomic configuration can be linear or circular, organized either as a single contiguous molecule or divided into multiple distinct segments.

  • Genomic Economy: Viral genomes are exceptionally compact compared to cellular genomes because they do not carry genes for metabolic pathways, membrane biogenesis, or organelle synthesis.

• Systematics and Classification:

  • Viruses are classified according to four primary criteria:

    • Type of nucleic acid ($DNA$ vs. $RNA$, $ss$ vs. $ds$).

    • Presence or absence of a lipid envelope (enveloped vs. naked).

    • Capsid shape and symmetry.

    • Physical virion size.

  • Taxonomic depth usually extends only to the family level; viral phylogenetic relationships remain incompletely understood because electron microscopy has existed for less than 100 years and comprehensive viral genomic sequencing is relatively recent.

• Representative Human Viruses by Genome Type:

  • DNA Viruses:

    • Herpesvirus ($dsDNA$, enveloped).

    • Smallpox virus ($dsDNA$, complex).

    • Parvovirus ($ssDNA$, naked; an uncommon example of a single-stranded DNA virus).

  • RNA Viruses:

    • Single-stranded RNA ($ssRNA$): Influenza virus, Human Immunodeficiency Virus ($HIV$), Norovirus, Common cold viruses (Rhinoviruses).

    • Double-stranded RNA ($dsRNA$): Rotavirus (a major cause of severe infantile gastroenteritis, currently preventable via childhood vaccination).

Bacteriophage Replication Cycles

• General Biology of Bacteriophages:

  • Bacteriophages (phages) are specialized viruses that exclusively infect bacterial cells.

  • Replicate via two primary pathways: the lytic cycle and the lysogenic cycle.

• The Lytic Replication Cycle:

  • Directs viral replication culminating in the rapid death and lysis of the host bacterial cell.

  • Exemplified by the T4 bacteriophage, a $dsDNA$ phage that specifically infects $Escherichia\,coli$.

  • Five Sequential Stages:

    1. Attachment (Adsorption):

       • Phages lack locomotive organelles (no flagella or cilia) and rely entirely on random Brownian motion to collide with bacterial cells.

       • Specialized tail fibers bind with high specificity to complementary proteinaceous receptors on the outer cell surface of $E.\,coli$.

    2. Entry (Penetration):

       • The virion releases lysozyme from its base, locally degrading the peptidoglycan cell wall (which is relatively thin in Gram-negative $E.\,coli$).

       • The tail sheath contracts, driving a hollow central core tube through the outer membrane and cell wall to inject the phage genome and associated enzymes directly into the cytoplasm.

       • The empty protein capsid remains attached to the exterior of the bacterium.

    3. Synthesis:

       • Host bacterial chromosomal DNA is degraded by viral enzymes.

       • The viral genome is transcribed into $mRNA$ and translated into viral structural proteins and enzymes, while host polymerases replicate copies of the viral DNA.

    4. Assembly (Maturation):

       • Capsomeres, baseplates, tail sheaths, and tail fibers assemble spontaneously or with chaperone assistance into complete capsids, encapsidating the newly synthesized viral genomes.

    5. Release (Lysis):

       • Endogenously produced lysozyme continuously degrades the peptidoglycan cell wall from within.

       • The compromised bacterial envelope succumbs to osmotic pressure, causing the cell to burst open and release all progeny virions simultaneously.

  • Kinetics: A single infecting virion can produce a burst size of approximately $100$ mature virions within a burst time of $25\text{ minutes}$.

• The Lysogenic Cycle (Lysogeny):

  • A modified, extended replication pathway in which host cell death is delayed.

  • Operates through temperate phages (also known synonymously as lysogenic phages, prophages, or inactive phages).

    • The proliferation of four distinct names for the same entity reflects historical, pre-internet scientific competition in the 1970s and 1980s, when isolated international research groups published findings simultaneously in print journals and promoted competing nomenclature at conferences.

  • Sequential Events of Lysogeny:

    • Attachment and Entry: Occur identically to the lytic cycle.

    • Genomic Integration: Instead of destroying the bacterial chromosome, the double-stranded viral DNA integrates directly into the host bacterial chromosome, becoming a prophage.

    • Passive Replication: Every time the host bacterium replicates its own chromosome prior to binary fission, it simultaneously copies the integrated prophage DNA, passing it on to all subsequent bacterial generations.

    • Lysogenic Conversion: Occurs when genes carried by the integrated prophage alter the physiological or pathogenic phenotype of the host bacterium.

    • Induction: An environmental stimulus (such as DNA damage, chemical mutagens, or physical stress) signals impending host cell compromise, prompting the prophage to excise from the bacterial chromosome. The excised viral genome degrades the bacterial chromosome and re-enters the lytic cycle (synthesis, assembly, and release).

  • Population-wide Release: Because the viral genome has multiplied within expanding bacterial lineages prior to induction, an environmental trigger induces vast populations of bacteria concurrently, producing an enormous, synchronized release of virions.

  • Evolutionary Advantage: Lysogeny permits the viral genome to hide safely inside the host chromosome, replicating silently and passively without triggering host immune/defense countermeasures or causing early host destruction.

Animal Virus Replication Strategies

• Distinctions from Bacteriophage Replication:

  • Host cells are eukaryotic, containing a membrane-bound nucleus and lacking rigid peptidoglycan cell walls.

  • Animal viruses lack tail fibers and baseplates; host attachment depends on glycoprotein spikes or capsid capsomeres binding to host plasma membrane receptors.

  • Presence of membranous envelopes in many animal viral families changes entry and exit mechanisms.

• Mechanisms of Viral Entry into Animal Cells:

  • Direct Penetration:

    • Characteristic of select naked (non-enveloped) viruses.

    • Viral capsid proteins bind to host cell receptors, inducing the formation of a pore in the plasma membrane through which the viral nucleic acid is injected directly into the cytosol, leaving the empty capsid outside the cell.

    • Example: Poliovirus.

  • Membrane Fusion:

    • Occurs exclusively among enveloped viruses.

    • Viral envelope glycoproteins bind host membrane receptors, prompting the viral lipid bilayer to fuse directly with the host plasma membrane.

    • The intact nucleocapsid is emptied into the host cytoplasm, requiring subsequent enzymatic uncoating to digest the capsid and release the genome.

    • Example: Measles virus.

  • Endocytosis:

    • Can be utilized by either enveloped or naked viruses.

    • Attachment of the virion stimulates receptor-mediated endocytosis, triggering the host cell to engulf the intact virion within an endocytic vesicle.

    • Once internalized, the vesicle must be breached, followed by capsid uncoating to free the viral genome into the cytoplasm.

    • Naked Virus Example: Adenovirus (which causes respiratory and liver infections).

    • Enveloped Virus Example: Herpes simplex virus.

• Subcellular Sites of Synthesis and Assembly:

  • DNA Viruses: Typically translocate to the host nucleus, where viral DNA replication and transcription occur; viral structural proteins are translated in the cytoplasm and transported back into the nucleus for virion assembly.

  • RNA Viruses: Typically execute genomic replication, protein translation, and assembly entirely within the host cytoplasm.

• Mechanisms of Genome Synthesis across Animal Virus Classes:

  • Double-Stranded DNA ($dsDNA$) Viruses:

    • Replication mirrors host cell genomic replication; viral $dsDNA$ enters the nucleus and is replicated by cellular or viral DNA polymerases, while transcription is mediated by RNA polymerases.

  • Single-Stranded DNA ($ssDNA$) Viruses:

    • Highly atypical because eukaryotic cells do not utilize single-stranded DNA genomes; host innate immune receptors recognize single-stranded DNA as foreign.

    • Replication Strategy: The viral $ssDNA$ folds back on itself into a hairpin loop to form a locally double-stranded DNA template, enabling host DNA polymerase to synthesize a complementary strand; this replicative form produces new single-stranded genomes for viral packaging.

    • Example: Parvovirus.

  • Positive-Sense Single-Stranded RNA ($+ssRNA$) Viruses:

    • The viral genomic RNA is identical in polarity to eukaryotic $mRNA$.

    • Upon entry into the cytoplasm, host ribosomes immediately recognize the genome and translate it directly into viral proteins.

    • Example: Poliovirus.

  • Negative-Sense Single-Stranded RNA ($-ssRNA$) Viruses:

    • The viral genome is complementary to $mRNA$ and cannot be directly translated by host ribosomes.

    • The virus must package an RNA-dependent RNA polymerase within the virion to transcribe the negative-sense strand into complementary positive-sense $mRNA$ before translation can proceed.

    • Examples: Rabies virus, Influenza virus.

  • Double-Stranded RNA ($dsRNA$) Viruses:

    • Composed of double strands; requires unique viral polymerases to transcribe mRNA from the genomic duplex.

    • Example: Rotavirus.

• Mechanisms of Viral Release and Pathogenesis:

  • Release Capacity: The absolute quantity of virions produced per cell depends on viral species, genome size, and the initial metabolic vitality of the host cell (healthier cells sustain viral production longer, yielding higher numbers of virions).

  • Budding:

    • Utilized by enveloped viruses.

    • Assembled capsids align along the inner surface of the host plasma membrane where viral glycoprotein spikes have been inserted.

    • The host membrane evaginates and pinches off, encasing the nucleocapsid within a new envelope.

    • Because budding does not cause immediate cell lysis, it leads to persistent infections characterized by continuous, prolonged viral shedding without immediate host cell destruction.

  • Exocytosis and Lysis:

    • Naked viruses exit host cells either through transport vesicles via exocytosis or by inducing complete membrane rupture and cytolytic cell death.

Viral Latency and Retroviruses

• Characteristics of Viral Latency:

  • Analogous to bacterial lysogeny; animal virus latency is a condition in which viruses remain dormant inside host tissues for extended periods, sometimes for years, without showing active viral replication or acute disease symptoms.

  • Viruses displaying this behavior are called latent viruses or proviruses.

  • Clinical Examples:

    • Varicella-Zoster Virus: Causes acute chickenpox during primary childhood infection, then enters a dormant latent state within sensory neural ganglia; reactivation decades later produces shingles.

    • Herpes Simplex Viruses.

    • Hepatitis B Virus ($HBV$).

    • Human Immunodeficiency Virus ($HIV$ / $AIDS$).

  • Permanence of Animal Proviruses:

    • A key distinction between lysogenic bacteriophages and animal proviruses is that once an animal provirus integrates into the host chromosomal DNA, the integration is permanent; there is no natural mechanism for excision.

• Biology of Retroviruses ($HIV$):

  • Classification: Enveloped, positive-sense single-stranded RNA viruses.

  • Unique Enzymatic Activity: Utilize reverse transcriptase to reverse the conventional flow of genetic information by transcribing viral $RNA$ into a complementary $dsDNA$ intermediate.

  • Target Specificity: Displays narrow tropism specifically for human $CD4^+$ helper T lymphocytes.

  • Infection Cycle:

    • Attaches and enters the helper T cell via endocytosis.

    • The viral capsid undergoes complete uncoating in the cytoplasm.

    • Reverse transcriptase synthesizes double-stranded DNA from the single-stranded viral RNA template.

    • The synthesized viral $dsDNA$ translocates into the nucleus and integrates into the host chromosome, becoming a permanent provirus.

    • The provirus directs the ongoing transcription of viral mRNAs, translation of viral polyproteins, assembly of new nucleocapsids, and shedding via budding.

Viral Oncogenesis and Cancer

• Regulation of Cellular Division:

  • Cellular division is strictly controlled by counterbalancing genetic pathways:

    • Proto-oncogenes: Genes that actively signal and promote cell division and tissue growth.

    • Tumor Suppressor Genes: Genes that repress, halt, or downregulate cell division and trigger cell cycle checkpoints.

  • Pathology of Dysregulation:

    • Neoplasia: Uncontrolled, abnormal cell division occurring within a multicellular animal.

    • Tumor: A localized mass or group of neoplastic cells dividing inappropriately.

    • Benign Tumors: Remain confined to their primary anatomical site; generally non-invasive and often surgically removable, though masses in enclosed critical regions (e.g., the lower brainstem) can be life-threatening.

    • Malignant Tumors (Cancer): Infiltrate surrounding tissues and invade lymph or blood vessels.

    • Metastasis: The active dissemination and seeding of malignant neoplastic cells from the primary anatomical site to distant secondary locations throughout the body.

• The Oncogene Concept:

  • Proto-oncogenes become oncogenes when they undergo inappropriate genetic activation, mutation, or hyperactivation, driving unconstrained proliferation.

  • Conversely, mutations that inactivate tumor suppressor genes remove cellular division brakes, perpetuating unchecked mitosis.

  • Mutagenic Triggers: Ultraviolet ($UV$) light, ionizing radiation, and chemical carcinogens induce genetic mutations that activate oncogenes or inactivate tumor suppressors.

• Viral Induction of Malignancies:

  • Viruses cause up to $25\%$ (one-quarter) of all recognized human cancers.

  • Mechanisms of Viral Carcinogenesis:

    • Direct Delivery: Viral genomes can carry functional oncogenes that integrate into host chromosomes.

    • Promoter Activation: Insertion of viral regulatory sequences can activate adjacent, quiescent host proto-oncogenes.

    • Insertional Inactivation: Integration of viral DNA can directly disrupt and disable functional host tumor suppressor genes.

  • Specific Viral-Associated Malignancies:

    • Burkitt's lymphoma.

    • Hodgkin's lymphoma.

    • Kaposi's sarcoma.

    • Cervical cancer: The overwhelming majority of human cervical cancer cases are caused by infection with high-risk strains of Human Papillomavirus ($HPV$).

Laboratory Cultivation of Viruses

• Obligate Intracellular Growth Requirement:

  • Viruses cannot replicate or form colonies on standard, cell-free nutrient agar petri dishes or synthetic broth media.

  • Viral cultivation strictly requires living, metabolically active host cells to support replication.

• Primary Cultivation Media:

  • Intact Mature Organisms: Inoculating living plants, laboratory animals, or bacterial cultures (e.g., cultivating bacteriophages on bacterial lawns to observe clearing plaques).

  • Embryonated Bird Eggs:

    • Fertile chicken eggs provide a sterile, self-sustaining, compartmentalized environment with diverse embryonic tissues.

    • Serves as the primary industrial substrate for propagating influenza virus for commercial vaccine manufacturing.

  • Cell Cultures (Tissue Cultures):

    • Isolated host cells propagated in vitro in sterile culture dishes containing specialized liquid growth media.

    • Permits controlled laboratory study of viral cytopathic effects and replication kinetics.

Subviral Agents: Viroids

• Physical and Molecular Characteristics:

  • Extremely small, circular single strands of infectious RNA ($ssRNA$).

  • Distinction from Viruses: Viroids completely lack a protein capsid and envelope; they exist purely as naked circular RNA molecules.

  • Host Range: Exclusively infect and cause disease in plant hosts; unable to infect animals or bacteria.

  • Agricultural Significance: Responsible for severe crop diseases, such as potato spindle tuber disease, causing marked stunting and tuber deformity.

• Mechanism of Pathogenesis:

  • Viroid RNA does not encode any viral proteins.

  • Pathogenicity occurs via complementary nucleotide base-pairing:

    • The viroid single-stranded RNA binds to complementary host plant $mRNA$ sequences, producing regions of double-stranded RNA ($dsRNA$).

    • Plant cells possess innate defense enzymes that recognize double-stranded RNA as foreign (viral) and degrade the entire duplex.

    • This degradation destroys essential host plant mRNAs, preventing protein translation and producing plant tissue pathology and disease.

Subviral Agents: Prions and Transmissible Spongiform Encephalopathies

• Fundamental Nature of Prions:

  • Proteinaceous infectious particles that lack nucleic acids entirely (devoid of both DNA and RNA).

  • Challenged traditional biological paradigms upon discovery due to their ability to transmit disease and propagate without a genetic genome.

• Conformational States of the Prion Protein ($PrP$):

  • Cellular $PrP$ ($c\text{-}PrP$):

    • Normal physiological membrane glycoprotein found in healthy mammalian tissues, particularly within the central nervous system.

    • Tertiary structure is predominantly folded into alpha-helices ($\alpha\text{-helices}$).

  • Prion $PrP$ ($p\text{-}PrP$):

    • Pathogenic, misfolded conformational isoform.

    • Possesses the identical primary amino acid sequence as $c\text{-}PrP$, but is refolded into a tertiary structure dominated by beta-pleated sheets ($\beta\text{-sheets}$).

  • Mechanism of Propagation:

    • When a pathogenic $p\text{-}PrP$ molecule encounters a normal $c\text{-}PrP$ molecule, it acts as a conformational template, binding the normal protein and inducing its $\alpha\text{-helical}$ regions to refold into aberrant $\beta\text{-sheets}$.

    • Newly converted $p\text{-}PrP$ molecules proceed to convert neighboring $c\text{-}PrP$ proteins in an exponential chain reaction.

• Neuropathology and Disease Presentation:

  • Misfolded $p\text{-}PrP$ forms insoluble amyloid fibrils and aggregates that disrupt neuronal physiology and lead to neuronal apoptosis.

  • Cell death leaves microscopic holes and large vacuoles throughout the cerebrum, causing the brain tissue to take on a porous, sponge-like histological appearance.

  • Collectively classified as Transmissible Spongiform Encephalopathies ($TSEs$).

  • Clinical Prognosis: Prion diseases are completely untreatable and universally fatal ($100\%$ mortality).

• Recognized Transmissible Spongiform Encephalopathies:

  • Scrapie: Affects sheep and goats.

  • Bovine Spongiform Encephalopathy ($BSE$ / "Mad Cow Disease"): Affects cattle; can cross into humans who ingest contaminated nervous or muscular tissues.

  • Chronic Wasting Disease ($CWD$):

    • Affects wild cervid populations (deer and elk).

    • Present in deer populations within Missouri, requiring hunters to submit deer heads for post-mortem testing.

    • Does not transmit to humans.

• Modes of Transmission:

  • Ingestion of contaminated animal tissues.

  • Tissue transplantation (e.g., contaminated dura mater grafts).

  • Direct contact of infectious material with mucous membranes.

  • latrogenic transmission via contaminated neurosurgical equipment.

• Extreme Resistance and Destruction Protocols:

  • Prions are resistant to standard cooking, environmental conditions, standard autoclaving, and routine clinical chemical disinfectants.

  • Thermal destruction requires extreme measures; standard culinary cooking temperatures do not affect prion infectivity.

  • Destruction Protocols:

    • Prolonged autoclaving for hours in a solution of concentrated sodium hydroxide ($NaOH$).

    • Complete high-temperature thermal incineration.

  • Biocontainment Strategies:

    • The European Union has approved specific enzymatic cleaning formulations to strip prions from reusable medical instrumentation.

    • In the United States, when an infected animal is identified within a domestic herd, veterinary health authorities cull and incinerate the entire herd to eliminate potential transmission pathways.

Questions & Discussion

• Evolutionary Advantages of Bacterial Lysogeny:

  • Question: Is there an evolutionary advantage for a bacteriophage to enter lysogeny other than yielding more virions at the end?

  • Response: Lysogeny provides an evolutionary sanctuary; by integrating its genome into the bacterial host chromosome, the virus avoids active host defense systems and environmental hazards, replicating passively alongside the host cell until favorable conditions trigger induction.

• Historical Eradication of Mad Cow Disease in the United Kingdom:

  • Question: How did England address and eliminate its massive outbreak of Bovine Spongiform Encephalopathy (Mad Cow Disease) in the 1990s?

  • Response: The United Kingdom addressed the outbreak through the mass culling and total thermal incineration of large numbers of cattle herds, combined with widespread, aggressive testing across their livestock population (a control measure made easier by the geography of a compact island nation compared to larger landmasses).

• Scope of Viral Pathogens Required for Study:

  • Question: Given the numerous virus examples mentioned in discussion, is there a specific comprehensive list students should memorize, or should focus be limited to class examples?

  • Response: Focus should be directed specifically to the explicit examples discussed in class (e.g., Herpes simplex virus, smallpox, parvovirus, influenza virus, $HIV$, norovirus, poliovirus, measles, adenovirus, rotavirus, rabies, and human papillomavirus), rather than attempting to memorize exhaustive taxonomy tables, which were shown only to illustrate classification categories.