Lesson 2 of 6 / Infectious diseases
Bacteria and viruses
Why does pathogen structure matter?
In this lesson: Compare a typical bacterium and virus, including their genetic material and reproduction.
About 5 min
The key ideaA bacterium is a cell with DNA not enclosed in a nucleus. A virus has genetic material inside a protein coat and replicates only in living host cells.
DNA without a nucleus: Genetic material lies in the cell, not enclosed in a membrane-bound nucleus.
Some bacteria are pathogenic; many are not. A typical bacterium can divide when conditions are suitable.
Dashed ring: selected feature. Shapes are original schematics, not to the same scale. The virus shown has no envelope; some viruses have an additional envelope.
Explanation
A typical bacterium has a cell wall outside its cell membrane, cytoplasm and ribosomes. Its main DNA is not enclosed in a nucleus. Bacteria are cells; many can reproduce by cell division when conditions are suitable.
Some bacteria are pathogenic, but many are not. Bacteria can play useful roles in decomposition, food production and the normal communities of microorganisms living in and on the body. The word bacterium does not automatically mean disease-causing.
A typical virus contains genetic material, either DNA or RNA, enclosed in a protein coat. Some viruses also have a surrounding envelope. A virus does not have the full cellular machinery needed to reproduce independently.
Viruses replicate only inside living host cells, using the host machinery to produce components that assemble into new viruses. They do not reproduce by bacterial cell division. Their lack of bacterial cell structures explains why an antibiotic aimed at a bacterial cell wall does not act on a virus.
Step by step
- 1
Identify cell organisation
A bacterium is a cell; a virus is not a small bacterial cell.
- 2
Locate the genetic material
Bacterial DNA is not enclosed in a nucleus. Viral genetic material is inside a protein coat.
- 3
Link structure to reproduction
A virus depends on living host-cell machinery, while bacteria divide as cells.
Worked example
Two schematic specimens
Specimen A has cytoplasm, ribosomes and DNA without a nucleus. Specimen B has genetic material inside a protein coat and only replicates inside living cells. Identify each.
One way to explain it
A is a bacterium. B is a virus. The cellular organisation of A and the protein coat plus host-cell dependence of B distinguish them.
Why this answer works
- Use several features together.
- Absence of a nucleus alone does not make A a virus.
- Host-cell dependence supports the identification of B.
Is this true? "A virus is a bacterium without a nucleus."
Typical bacteria themselves have no membrane-bound nucleus. Viruses have a different organisation: genetic material inside a protein coat and dependence on living host cells for replication.