Protein synthesis: transcription and translation

Transcription

1. In the nucleus, the DNA double helix unwinds and hydrogen bonds break over the length of the gene.

2. One strand, the template (antisense) strand, is used. Free RNA nucleotides pair with its exposed bases (A with U, T with A, C with G, G with C).

3. RNA polymerase joins the RNA nucleotides with phosphodiester bonds, forming mRNA.

4. mRNA leaves the nucleus through a nuclear pore to a ribosome.

The mRNA sequence is the same as the non-template (sense) strand, with U instead of T.

Translation

1. mRNA attaches to a ribosome, which reads it from the start codon (AUG).

2. Each tRNA carries a specific amino acid and has an anticodon, three bases complementary to an mRNA codon.

3. Two tRNAs bind to codons in the ribosome; a peptide bond forms between their amino acids. (Energy from ATP was used earlier, to attach each amino acid to its tRNA.)

4. The ribosome moves along one codon; the first tRNA leaves and is reused.

5. This repeats until a stop codon is reached; the polypeptide is released.

Molecules and their roles

Learn the job of each molecule in the table.

Worked example

A DNA template strand reads TAC GGA. Give the mRNA codons and the tRNA anticodons.

1. mRNA is complementary to the template: AUG CCU.
2. Anticodons are complementary to the codons: UAC GGA.

Answer: mRNA: AUG CCU; anticodons: UAC GGA.

Check yourself

1. A DNA template strand has the sequence TACGTT. Write the mRNA transcribed from it.
   Show answer

   AUGCAA. T pairs with A, A with U, C with G, G with C: AUG CAA.

2. An mRNA codon is GCU. What is the tRNA anticodon?
   Show answer

   CGA. Complementary to GCU: CGA.

3. Which enzyme makes mRNA during transcription?
   • DNA polymerase
   • RNA polymerase
   • DNA helicase only
   • Amylase
   Show answer

   RNA polymerase. RNA polymerase joins RNA nucleotides.

4. Where does translation take place?
   • In the nucleus
   • In the Golgi apparatus
   • At ribosomes in the cytoplasm or on rough ER
   • In the cell surface membrane
   Show answer

   At ribosomes in the cytoplasm or on rough ER. mRNA leaves the nucleus to be translated at ribosomes.

5. What is the role of tRNA?
   • To carry a specific amino acid to the ribosome and pair with a codon
   • To carry the genetic code out of the nucleus
   • To join RNA nucleotides
   • To unwind DNA
   Show answer

   To carry a specific amino acid to the ribosome and pair with a codon. tRNA's anticodon matches the mRNA codon.

6. What happens when the ribosome reaches a stop codon?
   • A new amino acid is added
   • Transcription starts
   • Translation ends and the polypeptide is released
   • The mRNA is copied
   Show answer

   Translation ends and the polypeptide is released. Stop codons do not code for an amino acid.

7. During transcription, which DNA strand is copied?
   • The template (antisense) strand
   • The two strands at the same time
   • Whichever strand has more adenine
   • The strand inside the ribosome
   Show answer

   The template (antisense) strand. Only the template strand is used; the mRNA matches the other (sense) strand except U for T.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• NCBI Bookshelf: Molecular Biology of the Cell, From RNA to Protein

Last updated 7 November 2026. Spotted a mistake? Tell us.