# Explain why a strand of mRNA produced during transcription is complementary to, but not identical to, the DNA template strand from which it was made, yet has the same base sequence as the non-template (coding) strand of DNA, with one exception. Identify that exception and explain its molecular basis.

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

> During gene expression, the enzyme RNA polymerase moves along one strand of DNA, the template strand, synthesising a single-stranded mRNA molecule. The mRNA then moves to ribosomes where it is translated into a polypeptide.

## Mark scheme (4 marks)

1. mRNA is complementary to the template strand because RNA polymerase adds free ribonucleotides by complementary base pairing with the template strand
2. Because the mRNA sequence is complementary to the template strand, it therefore has the same sequence as the non-template (coding) strand, since the two DNA strands are themselves complementary to each other
3. The exception is that mRNA contains uracil (U) where the coding strand of DNA contains thymine (T)
4. The molecular basis is that RNA nucleotides contain ribose sugar rather than deoxyribose, and uracil (not thymine) is the pyrimidine incorporated — uracil lacks the methyl group present on thymine but still base-pairs with adenine

## Key terms

- [mRNA](https://www.gradenine.co.uk/glossary/mrna)
- [template strand](https://www.gradenine.co.uk/glossary/template-strand)
- [transcription](https://www.gradenine.co.uk/glossary/transcription)
- [complementary](https://www.gradenine.co.uk/glossary/complementary)
- [non-template (coding) strand](https://www.gradenine.co.uk/glossary/non-template-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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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