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1.5.1Inquiry Process

Virus Structure — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 12 of 160

📚 IB Biology HL Hub / Topical Revision
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Part 1 of 5Parts of a Virus

1.5.1 Virus Structure

Viruses are tiny, usually 20 to 300 nm, and can only be seen with an electron microscope.

Genetic material
  • Each virus carries DNA or RNA, which may be single or double stranded, linear or circular depending on type.
Capsid
  • A protein coat built from repeating subunits that surrounds and protects the nucleic acid.
Envelope
  • In some viruses a lipid layer taken from the host membrane covers the capsid and carries glycoproteins.
Attachment proteins
  • Surface proteins that bind to specific receptors on host cells, deciding which cells can be infected.
Enzymes
  • Some viruses carry their own enzymes, such as reverse transcriptase in retroviruses like HIV.

Part 2 of 5Why Viruses Are Not Cells

Why Viruses Are Not Cells
No cytoplasm
  • Viruses have no cytoplasm, membrane-bound organelles or ribosomes, so they cannot make their own proteins.
No metabolism
  • They do not respire or grow independently and show activity only when inside a host cell.
Obligate parasites
  • They can reproduce only by using the machinery of a living host cell, so they are strictly parasitic.
Outside the domains
  • Because they are non-cellular, they do not fit into Archaea, Bacteria or Eukarya in the three-domain system.

Part 3 of 5Examples of Viral Structure

1
The bacteriophage has a head containing DNA, a tail and tail fibres that attach to a bacterial surface.
2
HIV is an enveloped virus whose genome is two copies of single-stranded RNA.
3
Influenza virus has an envelope with spike proteins and a segmented RNA genome.
4
The tobacco mosaic virus is a helical rod with RNA wound inside a protein tube.
5
Naked viruses lack the envelope and so depend on their capsid proteins alone for attachment.
VirusGenomeEnvelope
Bacteriophage T4dsDNANo
HIVssRNAYes
InfluenzassRNAYes

Part 4 of 5Worked Example

Question

  • A virus particle is 100 nm across and a bacterium is 2 μm across.
  • Calculate how many times larger the bacterium is, and state what instrument is needed to see the virus.

  1. Convert to the same unit2 μm = 2000 nm.
  2. Divide2000 ÷ 100 = 20.
  3. A virus is smaller than the resolving power of a light microscope (about 200 nm), so an electron microscope is needed.

Answer

  • The bacterium is 20 times larger;
    • an electron microscope is required to see the virus.

Part 5 of 5Exam Checklist & Tips

1.5.1 – Key Points for the Exam

  • Capsid The protein coat of a virus.
  • Envelope An outer lipid layer derived from the host cell membrane.
  • Obligate parasite An organism that can only survive by living inside a host.
  • Bacteriophage A virus that infects bacteria.

Common Mistakes

  • Calling viruses prokaryotes; they are non-cellular and sit outside the three domains.
  • Saying every virus has DNA; many have RNA.
  • Believing viruses are larger than bacteria; they are far smaller.
  • Describing a virus as made of a cell membrane; only enveloped types carry a lipid layer, taken from the host.

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