1.1. Classify viruses on basis of their structure/ number of strands/ diseases/ hosts etc.
〰️ Unit 1: Classification of Viruses
Chapter 1: Acellular Life
Student Learning Outcomes (SLO 1.1)
Learning Objectives
- Define the basic architectural components of a virion (genome, capsid, and envelope).
- Classify viruses based on morphological symmetry (helical, icosahedral, complex).
- Categorize viruses by their genomic architecture (DNA vs. RNA, single vs. double-stranded).
- Differentiate viruses based on host range and clinical disease (tropism).
📺 Video Lesson: Viral Taxonomy and Structure
An analytical overview of the criteria used by the International Committee on Taxonomy of Viruses (ICTV) to categorize viral entities.
1. Classification by Structure (Morphology)
Viruses are obligate intracellular parasites, typically ranging from $20 \, \text{nm}$ to $300 \, \text{nm}$ in size. They are primarily classified by the geometry of their capsid (the protein coat made of repeating subunits called capsomeres) and the presence or absence of a lipid envelope:
- Helical Symmetry: Capsomeres bind to the viral genome in a winding, spiral fashion, forming a hollow tube. Example: Tobacco Mosaic Virus (TMV), Rabies virus.
- Icosahedral (Polyhedral) Symmetry: A highly efficient geometric shape consisting of 20 equilateral triangular faces and 12 vertices. This structure maximizes internal volume to house the genome. Example: Adenovirus, Poliovirus.
- Complex Symmetry: Possess neither strictly helical nor strictly icosahedral symmetry, often combining both or having distinct structural elements like a head and a tail sheath. Example: T-even Bacteriophages, Poxviruses.
- Naked vs. Enveloped: Naked viruses consist only of the nucleocapsid. Enveloped viruses acquire a lipid bilayer from the host cell’s membrane during exocytosis, studded with viral glycoproteins (spikes). Example of Enveloped: Influenza, HIV, SARS-CoV-2.
2. Classification by Genetic Material (Genome Architecture)
Unlike cellular organisms, which strictly use double-stranded DNA (dsDNA), viral genomes display immense diversity. This is the basis of the Baltimore Classification System:
- DNA Viruses:
- Double-stranded DNA (dsDNA): Mimic host genomes. Examples: Herpesviruses, Poxviruses, Adenoviruses.
- Single-stranded DNA (ssDNA): Very rare. Example: Parvoviruses.
- RNA Viruses:
- Single-stranded RNA (ssRNA): The most common viral genome. Can be Positive-sense (+ssRNA), which acts directly as mRNA for immediate translation (e.g., Poliovirus, Hepatitis C), or Negative-sense (-ssRNA), which must be transcribed into +RNA by a packaged RNA-dependent RNA polymerase before translation (e.g., Rabies, Influenza).
- Double-stranded RNA (dsRNA): Example: Reoviruses (Rotavirus).
- Retroviruses: Contain +ssRNA but use the enzyme reverse transcriptase to convert their RNA into DNA, which then integrates into the host genome as a provirus. Example: HIV.
3. Classification by Host and Disease (Tropism)
Viruses are highly specific to their hosts due to the specific interaction between viral surface proteins and host cell receptors (viral tropism):
- Bacteriophages: Infect only bacteria (e.g., T4 phage).
- Plant Viruses: Enter through wounds or via vectors like insects (e.g., TMV).
- Animal Viruses: Clinically classified by the primary organ system they affect:
- Dermotropic: Affect the skin (Smallpox, Measles).
- Neurotropic: Affect the central nervous system (Rabies, Polio).
- Viscerotropic: Affect internal organs (Hepatitis viruses infect the liver).
- Pneumotropic: Affect the respiratory tract (Influenza, Rhinovirus).
🎯 MDCAT Exam Insights
- The Exceptions Rule: Medical entrance exams love exceptions. Memorize the outliers: Parvovirus is the only major ssDNA virus. Reovirus is the only major dsRNA virus. Poxvirus is a DNA virus but replicates exclusively in the cytoplasm (it brings its own DNA-dependent RNA polymerase).
- Sense vs. Antisense: In +ssRNA viruses, the genome is infectious on its own because host ribosomes recognize it as mRNA. Purified -ssRNA is not infectious on its own because the host lacks the enzyme to read it; the virus must carry pre-formed RNA-dependent RNA polymerase in its capsid.
📝 Concept Check
1. A researcher isolates a novel virus and determines that its purified genomic material alone is capable of directly binding to host ribosomes to initiate protein synthesis upon artificial injection into a cell. Based on this functional characteristic, how must this virus be classified?
As a negative-sense single-stranded RNA (-ssRNA) virus.
As a positive-sense single-stranded RNA (+ssRNA) virus.
As a Retrovirus.
Check Answer
Explanation: Positive-sense RNA genomes are structurally identical to cellular messenger RNA (mRNA). Therefore, they can be immediately translated by host ribosomes without the need for prior transcription by viral enzymes.
2. During an outbreak of a gastrointestinal disease, public health officials note that the causative viral agent remains highly stable on dry fomites (surfaces) and is entirely resistant to alcohol-based hand sanitizers. Which structural feature most likely accounts for this environmental resilience?
The absence of a lipid envelope (a naked nucleocapsid).
A complex helical morphology.
The presence of a reverse transcriptase enzyme.
Check Answer
Explanation: Alcohol sanitizers disrupt lipid bilayers. Enveloped viruses rely on their fragile lipid envelope for host attachment. Naked viruses possess only a tough, tightly packed protein capsid, making them highly resistant to desiccation, acidic environments (like the stomach), and lipid-solvents like alcohol.
3. Certain viruses are clinically classified as “neurotropic” because they specifically target and destroy neurons. Which fundamental biological mechanism determines this strict tissue specificity (tropism)?
The precise stereochemical match between viral surface glycoproteins and specific protein receptors on the host cell membrane.
The ability of the virus to cross the blood-brain barrier independently.
The size and geometric symmetry of the viral capsid.
Check Answer
Explanation: Viral tropism (host and tissue specificity) is governed entirely by attachment. A virus can only infect a cell if its surface ligands (spikes/glycoproteins) fit perfectly into specific receptor molecules present on the host cell’s surface, much like a key fits into a specific lock.
➡ Coming Next
Unit 2: Symptoms, Transmission, and Cause of Viral Disease (AIDS)
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