Describe how a molecule of messenger RNA (mRNA) is produced from a gene, and explain how ribosomes use this mRNA to assemble a polypeptide chain.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Protein synthesis occurs in two main stages: transcription and translation. Both stages rely on complementary base pairing.
Model answer (5 marks)
1. The DNA double helix unwinds and the two strands of the gene separate.
2. Free RNA nucleotides align along the template strand by complementary base pairing, forming the mRNA strand (transcription).
3. The mRNA molecule leaves the nucleus through a nuclear pore and travels to a ribosome.
4. The ribosome reads the mRNA in groups of three bases (codons) and tRNA molecules bring specific amino acids to the ribosome.
5. The amino acids are joined together in the specific sequence determined by the mRNA base sequence to form a polypeptide chain (translation).
2. Free RNA nucleotides align along the template strand by complementary base pairing, forming the mRNA strand (transcription).
3. The mRNA molecule leaves the nucleus through a nuclear pore and travels to a ribosome.
4. The ribosome reads the mRNA in groups of three bases (codons) and tRNA molecules bring specific amino acids to the ribosome.
5. The amino acids are joined together in the specific sequence determined by the mRNA base sequence to form a polypeptide chain (translation).
Examiner tips
- Use the exact terms ‘transcription’, ‘translation’, ‘codon’, ‘tRNA’, ‘ribosome’.
- Show the sequence of events: DNA unwinding → RNA synthesis → mRNA export → ribosome binding → codon reading → amino‑acid addition.
- Include the role of the nuclear pore and the ribosome’s reading frame.
- Keep each point brief and directly linked to the mark scheme.
Mark scheme (5 marks)
- The double helix of DNA unwinds / the two strands of the gene separate
- Free RNA nucleotides align along the template strand by complementary base pairing, forming the mRNA strand (transcription)
- The mRNA molecule leaves the nucleus through a nuclear pore and travels to a ribosome
- The ribosome reads the mRNA in groups of three bases (codons) / transfer RNA (tRNA) molecules bring specific amino acids to the ribosome
- Amino acids are joined together in a specific sequence (determined by the mRNA base sequence) to form a polypeptide chain (translation)
Key terms in this question
mRNA · ribosome · polypeptide
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
More DNA, RNA and protein synthesis questions
- A student learns that a mutation in a gene can sometimes result in a non-functio…
- A cell in the pancreas produces the protein insulin. Describe the roles of DNA, …
- A scientist compares the base sequence of a gene in a cell's nucleus with the ba…
- Explain how the sequence of bases in a gene determines the sequence of amino aci…