# A student examined two types of cell under an electron microscope and noted that one cell contained a nucleus, mitochondria, and ribosomes, while the other contained only ribosomes and a circular DNA molecule. Explain why the cell containing only ribosomes and a circular DNA molecule is classified as prokaryotic rather than eukaryotic.

> OCR A-Level Biology A (H420) — 2.1 Cell structure · Explain · 4 marks

## Mark scheme (4 marks)

1. Absence of a membrane-bound nucleus in the prokaryotic cell
2. DNA is circular and not associated with histone proteins
3. Absence of membrane-bound organelles in prokaryotic cells
4. Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S)

## Key terms

- [electron microscope](https://www.gradenine.co.uk/glossary/electron-microscope)
- [circular DNA molecule](https://www.gradenine.co.uk/glossary/circular-dna-molecule)
- [prokaryotic](https://www.gradenine.co.uk/glossary/prokaryotic)
- [eukaryotic](https://www.gradenine.co.uk/glossary/eukaryotic)
- [ribosomes](https://www.gradenine.co.uk/glossary/ribosomes)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-examined-two-types-of-3989f587) · Published by Druglandscape Ltd.