Viral Structure
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of prokaryotic and eukaryotic cell structures to understand the unique nature of viruses. You can test your knowledge on these below.
State one difference in the structure of DNA in prokaryotic cells and in eukaryotic cells.
Prokaryotic cells have a circular DNA molecule that is not associated with proteins (naked DNA), whereas eukaryotic cells have linear DNA associated with histones.
Name two organelles that are found in eukaryotic cells but absent in prokaryotic cells.
Mitochondria and endoplasmic reticulum (could also say nucleus / golgi body)
Where is the genetic material found in a prokaryotic cell?
The genetic material is located in the cytoplasm as a single circular DNA molecule, not enclosed in a nucleus.
Topic Explainer Videos
Check out this @LauraDoesBiology video that explains viral structure or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Overview of Viruses
- Viruses are acellular, they are not made from cells.
- Viruses cannot reproduce independently. They must infect a host cell to replicate.
- They do not carry out metabolic processes such as respiration.
- They are much smaller than prokaryotic and eukaryotic cells, typically 20–300 nm in size.
Structures Found in ALL Viruses
| Structure | Function |
| Genetic Material (DNA or RNA) | Contains the genetic material needed to code for viral proteins. |
| Capsid | A protein coat that protects the genetic material and aids in attachment to host cells. |
| Attachment Proteins | Glycoproteins on the capsid or lipid envelope that bind to receptors on host cells, allowing entry. |
Structures Found in SOME Viruses
| Structure | Function |
| Lipid Envelope | A phospholipid membrane surrounding the capsid, derived from the host cell membrane. Helps evade the immune system. |
| Enzymes (e.g., Reverse Transcriptase in Retroviruses) | Some viruses, like HIV, carry enzymes for replication inside the host cell. Reverse transcriptase allows viruses containing RNA to make DNA copies to infect a host cell. |
The Structure of HIV
Key Terms
- Virus: Acellular, non-living, infectious particle made of genetic material, a protein coat, and sometimes a lipid envelope.
- Capsid: A protein coat that surrounds and protects the viral genetic material.
- Attachment Protein: Surface proteins that allow the virus to bind to receptors on host cells.
- Lipid Envelope: A membrane derived from the host cell that surrounds some viruses.
- Retrovirus: A type of virus (e.g., HIV) that has RNA and uses reverse transcriptase to make DNA inside the host.
No answer provided.
Exam Tips
Students often confuse viruses with prokaryotic cells. Viruses are NOT cells – they have no nucleus, no ribosomes, no cytoplasm and no cell wall.
Not all viruses have a lipid envelope – only some do! The capsid is present in all viruses.
Don’t confuse the protein capsid found in all viruses with the slime capsule found in some prokaryotes.
No answer provided.
Describe the structure of a virus. (4 marks)
1. Viruses are acellular and non-living.
2. All viruses contain genetic material, which can be DNA or RNA.
3. The genetic material is enclosed in a protein coat called a capsid.
4. Viruses have attachment proteins, which allow them to bind to receptors on a host cell.
5. Some viruses have a lipid envelope.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!