# Explain why a restriction endonuclease and DNA ligase are both required when inserting a human gene into a bacterial plasmid vector.

> OCR A-Level Biology A (H420) — 6.4 Cloning and biotechnology · Explain · 4 marks

> Scientists use recombinant DNA technology to introduce human genes into bacterial cells so that the bacteria produce useful proteins, such as insulin, on an industrial scale. A key step involves cutting and joining DNA from two different organisms.

## Mark scheme (4 marks)

1. Restriction endonuclease cuts the DNA at a specific recognition sequence
2. Cutting with the same restriction endonuclease produces complementary sticky ends on both the plasmid and the human gene
3. Sticky ends allow the human gene to anneal/hydrogen bond to the open plasmid
4. DNA ligase seals the sugar-phosphate backbone to form a stable, covalently bonded recombinant plasmid

## Key terms

- [restriction endonuclease](https://www.gradenine.co.uk/glossary/restriction-endonuclease)
- [plasmid](https://www.gradenine.co.uk/glossary/plasmid)
- [vector](https://www.gradenine.co.uk/glossary/vector)
- [DNA ligase](https://www.gradenine.co.uk/glossary/dna-ligase)

## 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/explain-why-a-restriction-endonuclease-and-09f9aea4) · Published by Druglandscape Ltd.