Flu, polio, rabies and COVID-19 are all caused by viruses, yet these viruses are very different from each other. The book sorts them four ways: by what they infect, by their shape, by their genetic material and by how they copy themselves. Learn the four bases with one example each and most questions are covered.
Chapter 1 of the PECTAA Biology 11 book is Biodiversity and Classification. Section 1.6 covers the classification of viruses. This note is for FSc Part 1 (1st year, Class 11) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.
What to know
Why viruses stand apart. Viruses are acellular: they are not made of cells. They lack the features of the three domains of life, so they are not placed in any domain or kingdom, and they are not counted as living organisms. A virus is nucleic acid (DNA or RNA) inside a protein coat. It cannot run any metabolism of its own and depends on a host cell (plant, animal or bacterium) to replicate and make its proteins.
The book's side note adds two other acellular particles: prions, made of protein only, and viroids, made of circular RNA only. Both cause infectious diseases.
Virus classification follows guidelines from the International Committee on Taxonomy of Viruses (ICTV).
1. By host range.
- Animal viruses infect animals, including humans: influenza virus, rabies virus.
- Plant viruses infect plants: tobacco mosaic virus, potato virus X.
- Bacteriophages infect bacteria: T4 phage, lambda phage.
- Archaea viruses infect archaea: Sulfolobus spindle-shaped virus.
2. By morphology (shape).
- Helical: capsid with a helical structure around the nucleic acid. Tobacco mosaic virus, rabies virus.
- Icosahedral: capsid with symmetrical icosahedral shape. Adenoviruses, herpesviruses.
- Complex: a mix of icosahedral and helical features, sometimes with extra parts such as tails.
- Enveloped: an outer lipid envelope, taken from the host cell membrane, around the capsid. Influenza virus, HIV.
- Non-enveloped (naked): only a capsid around the nucleic acid. Poliovirus, adenovirus.
3. By genetic material.
- DNA viruses. Double-stranded (dsDNA): adenoviruses (respiratory infections), herpesviruses (herpes, chickenpox). Single-stranded (ssDNA): parvoviruses (gastroenteritis).
- RNA viruses. Single-stranded (ssRNA): coronaviruses (COVID-19), influenza viruses (flu). Double-stranded (dsRNA): rotaviruses (gastroenteritis).
4. By replication strategy.
- Positive-sense RNA viruses: the RNA genome is read directly by the host's ribosomes to make proteins. Poliovirus, Hepatitis C virus.
- Negative-sense RNA viruses: the RNA is first copied into mRNA by a viral RNA polymerase, then translated. Rabies virus, Ebola virus.
- Reverse transcribing viruses: replicate through a DNA intermediate using reverse transcriptase. RNA genome: retroviruses such as HIV (AIDS). DNA genome: hepadnaviruses such as Hepatitis B virus.
Key points
- One virus, several labels. Rabies virus is an animal virus, helical in shape and a negative-sense RNA virus. The bases are different ways of looking at the same virus.
- Adenovirus appears twice in shape lists: it is icosahedral and non-enveloped.
- Reverse transcriptase is the clue for HIV and Hepatitis B virus.
Quick revision table
| Basis | Groups | One book example each |
|---|---|---|
| Host range | animal, plant, bacteriophage, archaea | rabies virus, potato virus X, T4 phage, Sulfolobus spindle-shaped virus |
| Morphology | helical, icosahedral, complex, enveloped, naked | tobacco mosaic virus, herpesvirus, (tailed forms), HIV, poliovirus |
| Genetic material | dsDNA, ssDNA, ssRNA, dsRNA | adenovirus, parvovirus, influenza virus, rotavirus |
| Replication | positive-sense, negative-sense, reverse transcribing | Hepatitis C virus, Ebola virus, Hepatitis B virus |
Worked examples
Example 1. Classify influenza virus on three bases. Host range: animal virus. Morphology: enveloped. Genetic material: ssRNA.
Example 2. Why does a positive-sense RNA virus not need to make mRNA first? Its RNA genome already works as mRNA, so the host ribosomes translate it directly.
Example 3. A student says HIV is a DNA virus because it makes DNA. Correct this. HIV has an RNA genome. It makes a DNA intermediate using reverse transcriptase, which is why it is a reverse transcribing virus (a retrovirus).
Example 4. Name a virus that infects bacteria, and the group it belongs to. T4 phage or lambda phage. They are bacteriophages.
Common mistakes in the exam
- Calling viruses cells or organisms. They are acellular and are not classified in any domain or kingdom.
- Mixing prions and viroids. Prions are protein only; viroids are circular RNA only.
- Listing HIV as non-enveloped. The book lists it under enveloped viruses.
- Swapping positive-sense and negative-sense. Negative-sense RNA must be copied into mRNA first.
- Writing rotavirus as single-stranded. The book gives it as dsRNA.
Exam technique
- In "classify viruses" questions, name the basis first, then two types with one example each.
- Draw the five basic shapes (helical, icosahedral, complex, enveloped, non-enveloped) if a diagram is asked, as in the book's figure.
- Use the exact terms "capsid", "envelope" and "reverse transcriptase".
- Check your board's notice for the topics in your paper.
Practice questions
Which body sets the guidelines for virus classification? Answer: The International Committee on Taxonomy of Viruses (ICTV).
Give one example of a plant virus. Answer: Tobacco mosaic virus or potato virus X.
What is the genetic material of parvoviruses? Answer: Single-stranded DNA.
Name one negative-sense RNA virus. Answer: Rabies virus or Ebola virus.
What is the envelope of an enveloped virus made from? Answer: A lipid layer derived from the host cell membrane.
Why can a virus not grow on its own in a dish of nutrients? Answer: It has no metabolism of its own. It needs a host cell to replicate and to make its proteins.
Ready to practise this? Continue in 1st Year Biology.