A scientist compares the base sequence of a gene in a cell's nucleus with the base sequence of the messenger RNA (mRNA) produced from that gene. The scientist notices that the mRNA sequence is complementary to the template strand of the gene but is not identical to it. Explain why the mRNA sequence is complementary to the template strand, and describe what happens to the mRNA after it leaves the nucleus.

Pearson Edexcel A-Level Biology A (9BI0) — 1.4 DNA, RNA and protein synthesis · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Genes are found in the nucleus of a cell and are made of DNA. Messenger RNA (mRNA) is made using the gene as a template and carries the genetic code to the ribosomes.

Model answer (5 marks)

mRNA is synthesised by RNA polymerase using the DNA template strand as a guide – each base on the template pairs with its complementary base in the RNA (A with U, T with A, C with G, G with C). Therefore the mRNA sequence is complementary to the template strand and not identical to the coding strand.

After transcription the mRNA exits the nucleus through the nuclear pore complex, enters the cytoplasm and binds to a ribosome. The ribosome reads the mRNA codons and, using tRNA, joins the corresponding amino acids in the correct order to build a protein (polypeptide).

Examiner tips

  • Show the base‑pairing rule (A‑U, C‑G) to justify complementarity. Explain the transport through the nuclear pore and ribosome binding. Mention the ribosome’s role in translating codons into amino acids. Use the exact terminology: template strand, complementary, nuclear pore, ribosome, polypeptide.

Common mistakes

  • Confusing the coding strand with the template strand. Forgetting to mention the nuclear pore or ribosome. Using DNA bases (T) in the mRNA sequence instead of U.

Mark scheme (5 marks)

  1. mRNA is built by complementary base pairing with the template strand of the DNA
  2. Each base on the template strand of DNA pairs with a specific complementary base (e.g. A pairs with U, C pairs with G) to form the mRNA
  3. mRNA moves out of the nucleus (through the nuclear pore / nuclear membrane)
  4. mRNA moves to a ribosome (in the cytoplasm)
  5. The ribosome reads the base sequence of the mRNA and uses it to join amino acids in the correct order to make a protein / polypeptide

Key terms in this question

messenger RNA (mRNA) · template strand

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