State the features of viruses, limited to a protein coat and genetic material.

Cambridge IGCSE Biology (0610) – Topic 1.3: Features of Viruses

Core Objective

State the two permanent structural components of a virus and describe their key characteristics.

1. Protein Coat (Capsid)

• Composition: Made entirely of protein sub‑units called capsomeres.
• Function: Protects the viral nucleic acid from physical damage and from host enzymatic degradation.
• Shape determination: The arrangement of capsomeres gives the virion its external shape. The three common shapes are:
  • Icosahedral – 20 triangular faces (e.g., Adenovirus)
  • Helical – rod‑shaped, formed by a spiral of capsomeres (e.g., Tobacco mosaic virus)
  • Complex – capsid plus additional structures such as a tail (e.g., Bacteriophage T4)
• Visibility: The capsid is the only part of a virus that can be seen with an electron microscope.
• Supplementary note (extended content): Some viruses also possess an outer lipid envelope derived from the host cell membrane. The envelope is **not** part of the core definition of a virus in the IGCSE syllabus but is required for the extended (supplementary) objectives.

2. Genetic Material

• Type: Either DNA or RNA, never both.
• Physical form: May be single‑stranded (ss) or double‑stranded (ds); may be linear or circular.
• Role: Carries the information needed to:
  • Direct synthesis of viral proteins (capsid proteins, enzymes, etc.)
  • Direct replication of the genome inside a suitable host cell
• Classification relevance: The nature of the nucleic acid (DNA vs. RNA, ss vs. ds, linear vs. circular) is the primary basis for virus classification in the core syllabus.
• Supplementary note: In the extended syllabus, the number of genome segments (e.g., segmented RNA of Influenza virus) is also used for classification.

Why Only These Two Components Are Required by the Syllabus

• Viruses lack a nucleus, cytoplasm, cell membrane and all other organelles found in living cells.
• Because they contain only a protein coat and nucleic acid, they cannot carry out metabolism or reproduce independently; they must infect a suitable host cell and use the host’s biochemical machinery.

Summary Table

Illustrative Diagram

Practical / Experimental Skill (AO3)

• Activity: “Model a virus capsid using coloured paper cut‑outs for capsomeres and a strand of yarn for nucleic acid.”
• Learning outcomes:
  • Identify capsomere arrangement and relate it to virion shape.
  • Explain how the model demonstrates protection of genetic material.
• Safety note: When discussing real viruses, remind students that work with infectious viruses requires Biosafety Level 2 (BSL‑2) containment; in the classroom only simulated or non‑pathogenic models should be used.

Mathematical Example (AO2)

Question: A virus particle appears 0.2 mm long on a photograph taken with a magnification of × 50 000. What is the actual length of the virus in nanometres (nm)?

Solution:

1. Actual size = (Observed size) ÷ (Magnification) = 0.2 mm ÷ 50 000 = 4 × 10⁻⁶ mm
2. Convert to metres: 4 × 10⁻⁶ mm = 4 × 10⁻⁹ m
3. Convert to nanometres: 4 × 10⁻⁹ m × 10⁹ nm / m = 4 nm

Thus the virus is 4 nm in diameter – typical of small icosahedral viruses.

Key Points to Remember (AO1)

1. Viruses are non‑cellular; they lack a nucleus, cytoplasm and organelles.
2. The only permanent structural parts are the protein coat (capsid) and the nucleic acid.
3. The capsid protects the genome and determines the shape of the virion.
4. The genome may be DNA or RNA, single‑ or double‑stranded, linear or circular.
5. Because they have no metabolic machinery, viruses must enter a host cell to reproduce.
6. Classification in the core syllabus is based on nucleic‑acid type and capsid shape; the extended syllabus adds envelope status, genome segmentation, and replication strategy.

Exam‑Style Questions (AO2/3)

1. Explain why a virus cannot be classed as a living organism.
   • State that it lacks cellular structure and metabolic processes.
   • Emphasise that it contains only a protein coat and genetic material and must use a host cell’s machinery to reproduce.
   • Link this to the two permanent components described above.
2. Label the diagram of a virus. (Provide a blank version of the schematic above.)
   • Label: capsid, capsomeres, nucleic acid, (optional for extended) envelope.
3. Classify the following viruses according to the core syllabus criteria (DNA‑virus or RNA‑virus, and capsid shape):
   • Influenza virus – RNA, enveloped, helical (core: RNA‑virus; shape: helical)
   • Herpes simplex virus – DNA, enveloped, icosahedral (core: DNA‑virus; shape: icosahedral)
   • Bacteriophage T4 – DNA, non‑enveloped, complex (core: DNA‑virus; shape: complex)

Safety Box (BSL‑2 Reminder)