AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
It is a single DNA loop and there may be one or more small rings of DNA…
Full specification wording
Eukaryotes and prokaryotes: It is a single DNA loop and there may be one or more small rings of DNA called plasmids.
Prokaryotic cells, such as bacteria, store their genetic material differently from eukaryotic cells.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 5 of 6
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Prokaryotic cells, such as bacteria, store their genetic material differently from eukaryotic cells. Their main chromosome is one circular molecule of DNA, often described as a single DNA loop, which sits free in the cytoplasm rather than inside a nucleus. In addition, many bacteria carry plasmids: small circular rings of DNA that are separate from the main loop. Plasmids usually carry extra genes, for example for antibiotic resistance, and can be passed between bacteria. When you label a bacterial cell, show the loop and any plasmid rings clearly, and remember that both are DNA.
What earns the marks
- The main genetic material of a prokaryotic cell is a single DNA loop, not a nucleus.
- The DNA loop is circular and lies free in the cytoplasm.
- Plasmids are small rings of DNA, separate from the main DNA loop.
- A prokaryotic cell may contain no plasmids, one plasmid or several plasmids.
- Plasmids carry additional genes and can be transferred between bacteria.
Common mistakes and how to put them right
Saying the DNA loop is enclosed in a nucleus.
Put it right by stating that prokaryotes have no nucleus, so the loop lies free in the cytoplasm.
Describing plasmids as a protein coat or as food stores.
Put it right by identifying plasmids as small rings of DNA.
Assuming every bacterium has exactly one plasmid.
Put it right by saying there may be one or more plasmids, or none at all.
Exam tips
- Sketch a bacterium with one large loop and two small rings, then label each as DNA.
- Use the phrase single DNA loop when describing the main chromosome of a prokaryote.
- If a question asks why plasmids matter, link them to carrying extra genes such as antibiotic resistance.
Try the lesson
The start of the Eukaryotes and prokaryotes: It is a single DNA loop and there may be one or more small rings of DNA called plasmids. lesson, made from checked teaching notes for this specification point.
Have a go first
How is the main genetic material organised inside a bacterial cell?
Listen
Prokaryotic cells, such as bacteria, store their genetic material differently from eukaryotic cells. Their main chromosome is one circular molecule of DNA, often described as a single DNA loop, which sits free in the cytoplasm rather than inside a nucleus. In addition, many bacteria carry plasmids: small circular rings of DNA that are separate from the main loop. Plasmids usually carry extra genes, for example for antibiotic resistance, and can be passed between bacteria. When you label a bacterial cell, show the loop and any plasmid rings clearly, and remember that both are DNA. A strong answer earns marks for these points. The main genetic material of a prokaryotic cell is a single DNA loop, not a nucleus. The DNA loop is circular and lies free in the cytoplasm. Plasmids are small rings of DNA, separate from the main DNA loop. A prokaryotic cell may contain no plasmids, one plasmid or several plasmids. A common slip: Saying the DNA loop is enclosed in a nucleus. Instead, stating that prokaryotes have no nucleus, so the loop lies free in the cytoplasm. Exam tip: Sketch a bacterium with one large loop and two small rings, then label each as DNA.
Key words
Say what each word means before you turn the card.
DNA loop
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Plasmids
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See every definition
- DNA loop
- The main genetic material of a prokaryotic cell, which is a single DNA loop that lies free in the cytoplasm rather than inside a nucleus.
- Plasmids
- Small rings of DNA, separate from the main DNA loop, of which there may be one or more, or none at all.
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