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Translation: turning a message into a working protein questions

The structure of the ribosome and its two tRNA binding sites, tRNA anticodons, the three stages of translation, peptide bond formation, the ATP cost of protein synthesis, post-translational modification in the Golgi apparatus, and a worked route from a DNA sequence through mRNA to an amino acid sequence.

6 original questions · 21 marks · the translation: turning a message into a working protein notes · Nucleic acids, genomes and protein synthesis

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• 5 marks · Describe

  Describe what happens at a ribosome from the moment a second tRNA arrives until the ribosome has moved along the mRNA by one codon.

  Mark scheme

  1. B1 a tRNA whose anticodon is complementary to the codon in the A site binds there, held by hydrogen bonds between the paired bases
  2. B1 that tRNA carries the one specific amino acid corresponding to its anticodon
  3. B1 the two amino acids are now adjacent, and the ribosome catalyses the formation of a peptide bond between them by condensation
  4. B1 the first tRNA, now without its amino acid, leaves the ribosome
  5. B1 the ribosome moves along the mRNA by exactly one codon, so the remaining tRNA shifts into the P site and the A site is left free

• 4 marks · Explain

  Explain why a plasma cell, which secretes large quantities of antibody, contains a great many mitochondria.

  Mark scheme

  1. B1 the cell synthesises protein continuously and in very large quantities, so translation runs constantly
  2. B1 ATP is used to attach each amino acid to its tRNA, by the specific enzyme for that amino acid
  3. B1 further nucleoside triphosphates are hydrolysed to bring each tRNA into the ribosome and to move the ribosome along, roughly four high-energy bonds per amino acid added
  4. B1 mitochondria supply that ATP by aerobic respiration, so many are needed to keep pace with the demand

• 4 marks · Suggest

  Some antibiotics stop bacterial protein synthesis by binding to 70S ribosomes, and have no effect on the 80S ribosomes in human cytoplasm. Suggest why such drugs can nevertheless produce side effects in the patient.

  Mark scheme

  1. B1 mitochondria contain their own ribosomes, and these are 70S rather than 80S
  2. B1 the antibiotic can therefore bind to the ribosomes inside the patient's own mitochondria
  3. B1 protein synthesis within the mitochondria is inhibited, so the proteins they need cannot be made
  4. B1 aerobic respiration and the supply of ATP fall, which affects the tissues that demand most ATP

• 3 marks · Calculate

  The coding sequence of a mature mRNA molecule contains 927 bases and ends with a stop codon. Calculate the number of amino acids in the polypeptide produced and the number of peptide bonds within it.

• 3 marks · Describe

  Describe what happens to a polypeptide destined for secretion after it has been released from the ribosome.

• 2 marks · Name

  Name the site on a ribosome that holds the tRNA carrying the growing polypeptide chain, and name the bond formed between two adjacent amino acids.

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