# Explain why a restriction endonuclease and DNA ligase are both essential when inserting a human gene into a bacterial plasmid to produce a recombinant plasmid.

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

> Genetic engineers routinely use bacterial plasmids as vectors to introduce human genes into bacterial cells. This allows bacteria to produce human proteins such as insulin on an industrial scale.

## Mark scheme (4 marks)

1. Restriction endonuclease cuts the plasmid (and the donor DNA) at specific recognition sequences
2. This produces complementary sticky ends on both the plasmid and the human gene fragment
3. The complementary sticky ends allow the human gene to anneal/hydrogen bond into the plasmid
4. DNA ligase seals the sugar-phosphate backbone by forming covalent/phosphodiester bonds, permanently joining the gene into the plasmid

## Key terms

- [restriction endonuclease](https://www.gradenine.co.uk/glossary/restriction-endonuclease)
- [plasmid](https://www.gradenine.co.uk/glossary/plasmid)
- [recombinant plasmid](https://www.gradenine.co.uk/glossary/recombinant-plasmid)
- [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-7df19164) · Published by Druglandscape Ltd.