# 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

> Genetic engineers routinely transfer genes from human cells into bacterial plasmids so that bacteria can be used to manufacture human proteins such as insulin on an industrial scale.

## Mark scheme (4 marks)

1. Restriction endonuclease cuts the DNA at specific recognition sequences
2. This produces sticky ends (complementary/unpaired single-stranded overhangs) on both the plasmid and the human gene fragment
3. Complementary sticky ends allow the human gene and plasmid to anneal/base-pair by hydrogen bonding
4. DNA ligase seals/joins the sugar-phosphate backbone, forming a stable recombinant plasmid

## Key terms

- [restriction endonuclease](https://www.gradenine.co.uk/glossary/restriction-endonuclease)
- [plasmid](https://www.gradenine.co.uk/glossary/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-fa59155f) · Published by Druglandscape Ltd.