A virus is a tiny particle made of genetic material inside a protein coat. It is not a cell, and it can reproduce only inside a living host cell. This lesson in living systems and classification teaches you to compare structures accurately and to stop at the facts your syllabus expects.
What is inside a virus, compared with cells?
A cell has a cell membrane and cytoplasm that holds the chemical reactions of the cell. A virus has neither.
| Feature | Animal cell | Bacterial cell | Virus |
|---|---|---|---|
| Nucleus | Yes | No, DNA free in cytoplasm | No |
| Cell membrane | Yes | Yes | No |
| Cytoplasm | Yes | Yes | No |
| Cell wall | No | Yes | No |
| Genetic material | DNA in nucleus | DNA, including a loop and plasmids | DNA or RNA, inside a protein coat |
| Reproduces alone | Divides as part of an organism | Divides alone | No, needs a host cell |
The table is a summary. Check your own notes for diagrams and labels that your teacher wants you to know.
How does a virus reproduce?
A virus attaches to a host cell and its genetic material enters the cell. The host cell’s own machinery then makes new virus particles.
The virus particles leave and can infect other cells. The key point for an answer: the host cell does the work. The virus supplies the instructions.
Worked example
Question: State two differences between a bacterium and a virus.
Step 1, choose clear features from the table. Good choices are the cell membrane and the method of reproduction.
Step 2, write each difference as a full comparison.
- A bacterium has a cell membrane and cytoplasm, but a virus has neither.
- A bacterium can reproduce on its own, but a virus can reproduce only inside a host cell.
Step 3, check the marks. Each statement names both organisms. Writing only “a virus has no cytoplasm” does not compare, so it may not earn the mark.
The mistake to watch for
Mistaken answer: “A virus is the smallest type of cell, and it is alive because it reproduces.”
This makes two claims that are outside the facts: a virus is not a cell, and reproducing only inside a host cell is not the same as carrying out life processes alone.
The correction is to state what the structure shows: a virus is a non-cellular particle, and it needs a host cell to reproduce. If a question asks whether viruses are living, use the facts in your syllabus notes and avoid claims that the question did not ask for.
Check yourself
1. Name the two parts of a typical virus.
Show answer
Genetic material (DNA or RNA) and a protein coat.
2. A student labels a diagram of a virus with “nucleus” and “cytoplasm”. Explain the error.
Show answer
A virus is not a cell, so it has neither a nucleus nor cytoplasm.
3. Why can a virus not reproduce in a sterile nutrient broth, when a bacterium can?
Show answer
A virus needs a living host cell to make new virus particles. A sterile broth contains no host cells, but a bacterium can divide there because it is a complete cell.
Where this leads next
When you have finished this lesson, test the whole module with the living systems and classification practice set. For how cells are built, continue to cells and microscopy.
A teacher in online one-to-one Biology tuition can go through your own notes and help you decide how far to take a virus answer.