# A molecule of messenger RNA (mRNA) is produced during transcription. Explain how the sequence of bases in a gene determines the primary structure of a polypeptide, referring to the roles of mRNA and transfer RNA (tRNA) in this process.

> OCR A-Level Biology A (H420) — 2.3 Nucleotides and nucleic acids · Explain · 4 marks

## Mark scheme (4 marks)

1. The base sequence of the gene (template strand) is transcribed to produce a complementary mRNA sequence, so the sequence of codons on mRNA reflects the original DNA base sequence
2. Each codon (triplet of bases) on the mRNA codes for a specific amino acid
3. tRNA molecules carry specific amino acids and have a complementary anticodon that base-pairs with the mRNA codon at the ribosome
4. Amino acids are joined by peptide bonds in a sequence determined by the mRNA, forming the primary structure of the polypeptide

## Key terms

- [messenger RNA (mRNA)](https://www.gradenine.co.uk/glossary/messenger-rna-mrna)
- [transcription](https://www.gradenine.co.uk/glossary/transcription)
- [transfer RNA (tRNA)](https://www.gradenine.co.uk/glossary/transfer-rna-trna)
- [primary structure](https://www.gradenine.co.uk/glossary/primary-structure)

## 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-molecule-of-messenger-rna-mrna-ca2ac79a) · Published by Druglandscape Ltd.