# Explain why a molecule of messenger RNA (mRNA) produced during transcription is described as complementary to the template strand of DNA, and explain why the base sequence of mRNA is identical to the coding strand of DNA rather than complementary to it.

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

> During gene expression, a gene is first transcribed into mRNA before translation occurs. The DNA double helix has two strands: the template strand, which is read by RNA polymerase, and the coding strand (also called the sense strand), which has the same sequence as the mRNA produced.

## Mark scheme (4 marks)

1. RNA polymerase reads the template strand and adds nucleotides by complementary base pairing
2. Adenine pairs with uracil (and cytosine with guanine), so each base in mRNA is the complement of the corresponding base on the template strand
3. The coding strand has the same base sequence as the template strand's complement, so it shares the same sequence as the mRNA (with T replaced by U)
4. Therefore the mRNA carries the same genetic information as the coding strand, allowing the correct amino acid sequence to be specified during translation

## Key terms

- [messenger RNA (mRNA)](https://www.gradenine.co.uk/glossary/messenger-rna-mrna)
- [template strand](https://www.gradenine.co.uk/glossary/template-strand)
- [transcription](https://www.gradenine.co.uk/glossary/transcription)
- [coding strand](https://www.gradenine.co.uk/glossary/coding-strand)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-molecule-of-messenger-c8224bfb) · Published by Druglandscape Ltd.