Lesson 5 of 7 / Cells and stem cells
A bacterium is a cell without a nucleus
What does a bacterium need to make proteins?
In this lesson: Describe the defining structures of a typical bacterial cell.
About 4 min
The key ideaA typical bacterium is small and unicellular, with circular DNA, 70S ribosomes and a peptidoglycan wall, but no membrane-bound nucleus or other membrane-bound organelles.
Explore the idea
A cell without a nuclear envelope
Bacteria synthesise proteins on 70S ribosomes. These are not membrane-bound organelles.
Original schematic. Structures, cell numbers and distances are illustrative, not to scale.
Explanation
Bacteria are cellular organisms. A typical bacterial cell has cytoplasm, a cell surface membrane and a wall made of peptidoglycan. Its wall is chemically different from a plant cellulose wall. It is generally smaller than a typical eukaryotic cell.
The main bacterial chromosome is circular DNA located in a region of the cytoplasm, without a nuclear envelope. Smaller circular plasmids may also occur, but not every bacterium has the same plasmids. The cell has 70S ribosomes for protein synthesis.
Lack of membrane-bound organelles does not mean lack of metabolism. Bacterial enzymes and membranes support essential reactions without mitochondria, chloroplasts, ER or Golgi bodies. Do not label a bacterium as a virus simply because it lacks a nucleus.
Step by step
- 1
Find shared cell features
Both bacterial and eukaryotic cells possess membranes, cytoplasm, DNA and ribosomes.
- 2
Find the distinguishing organisation
Look for DNA without a nuclear envelope and absence of membrane-bound organelles.
Worked example
An unknown single cell
A small cell has a peptidoglycan wall, circular DNA and 70S ribosomes. Is the absence of a nucleus evidence that it cannot synthesise proteins?
One way to explain it
No. These features fit a typical bacterial cell. Its DNA provides genetic information and its 70S ribosomes synthesise polypeptides; a membrane-bound nucleus is not required.
Why this answer works
- Use the full set of features.
- Distinguish a missing compartment from a missing process.
Is this true? "Bacteria contain no organelles, so they have no ribosomes."
They lack membrane-bound organelles but possess non-membrane-bound 70S ribosomes.