Questions on translation and protein synthesis

Question 1

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

Accepted Answer

B1 a tRNA whose anticodon is complementary to the codon in the A site binds there, held by hydrogen bonds between the paired bases B1 that tRNA carries the one specific amino acid corresponding to its anticodon B1 the two amino acids are now adjacent, and the ribosome catalyses the formation of a peptide bond between them by condensation B1 the first tRNA, now without its amino acid, leaves the ribosome 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

Question 2

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

Accepted Answer

B1 the cell synthesises protein continuously and in very large quantities, so translation runs constantly B1 ATP is used to attach each amino acid to its tRNA, by the specific enzyme for that amino acid 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 B1 mitochondria supply that ATP by aerobic respiration, so many are needed to keep pace with the demand

Question 3

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.

Accepted Answer

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

Question 4

Describe what happens during the initiation and the termination of translation, naming the codons involved.

Accepted Answer

B1 in initiation, mRNA binds to the small ribosomal subunit, and the ribosome moves along the mRNA until it reaches the start codon, AUG B1 a tRNA with the complementary anticodon, UAC, carrying methionine, binds in the P site, and the large ribosomal subunit then joins to complete the ribosome B1 in termination, translation ends when one of the three stop codons, UAA, UAG or UGA, reaches the A site, and no tRNA has a matching anticodon to bind there B1 a release factor binds instead, the completed polypeptide is released, and the ribosome separates into its two subunits

Question 5

Preproinsulin is a single polypeptide chain of 110 amino acids. A signal sequence of 24 amino acids is removed as the chain enters the endoplasmic reticulum, giving proinsulin. A connecting section of 35 amino acids in total is then cut out of proinsulin, leaving the two chains of the active hormone joined by disulfide bridges. Calculate the number of amino acids in proinsulin, and calculate the number of amino acids in the finished, active insulin molecule.

Accepted Answer

M1 proinsulin = preproinsulin − signal sequence = 110 − 24 A1 86 amino acids in proinsulin M1 active insulin = proinsulin − connecting section removed = 86 − 35 A1 51 amino acids in the finished, active insulin molecule

Question 6

Explain why a cell regulates transcription of a gene rather than transcribing it continuously, and explain why switching transcription off stops production of its protein within hours.

Accepted Answer

B1 producing an mRNA molecule that will be translated many times consumes ATP whether or not the protein is actually needed by the cell at that moment, so continuous, unregulated transcription would be wasteful B1 mRNA molecules have a short lifetime compared with many proteins, being broken down within minutes to hours rather than persisting indefinitely B1 once transcription of a gene stops, no new mRNA for that gene is produced, and the existing molecules continue to degrade at their usual rate rather than being replaced B1 translation of that protein therefore falls as the supply of its mRNA runs out, so production stops within hours of the gene being switched off, rather than continuing for as long as the protein itself persists

Question 7

A student states that tRNA carries the codon to the ribosome, where it is matched against the mRNA. Discuss whether this description correctly identifies which molecule carries which.

Accepted Answer

B1 what is right: the student is correct that matching occurs at the ribosome between a triplet on tRNA and a triplet on mRNA, held together by hydrogen bonds between complementary bases B1 against: it is the mRNA, not the tRNA, that carries the codon; the codon is the triplet on the mRNA that specifies an amino acid or a stop signal B1 against: the tRNA carries the anticodon, a triplet complementary to the codon, together with the one specific amino acid corresponding to that anticodon, and it is this anticodon that pairs with the mRNA's codon rather than the codon being carried by the tRNA B1 judgement: reversing which molecule carries which is not a minor slip, since codon and anticodon are defined by which strand they are on; stating that tRNA 'carries the codon' describes mRNA's role, not tRNA's

Question 8

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.

Accepted Answer

M1 divides the number of bases by three to give the number of codons: 927 ÷ 3 = 309 M1 subtracts the stop codon, which specifies no amino acid A1 308 amino acids, joined by 307 peptide bonds

Question 9

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

Accepted Answer

B1 it folds into its secondary and tertiary structure, a process that begins as the chain emerges and is completed in the endoplasmic reticulum B1 vesicles carry it to the Golgi apparatus, where post-translational modification takes place B1 carbohydrate or lipid groups may be added and sections may be cut out, and the finished protein is packaged into a vesicle for export

Question 10

A ribosome has three tRNA-binding positions, called X, Y and Z here. Position X holds the tRNA carrying the growing polypeptide chain. Position Y accepts the next tRNA, whose anticodon must match the codon there. Position Z is occupied briefly by an unloaded tRNA before it leaves the ribosome. Describe the order in which a single tRNA molecule occupies X, Y and Z during translation, and describe what it carries at each position.

Accepted Answer

B1 it enters at Y, carrying its one specific amino acid, with its anticodon paired to the codon there B1 once a peptide bond has joined the growing chain to its amino acid it carries the whole chain, and when the ribosome moves along one codon it is in X B1 after the chain has been passed to the amino acid on the next tRNA it carries nothing, and the next move brings it to Z, from which it leaves to collect another molecule of its amino acid

Question 11

Explain how the specific enzyme that attaches an amino acid to a tRNA molecule ensures that the genetic code is correctly translated.

Accepted Answer

B1 there is a separate activating enzyme for each amino acid, and each one recognises and attaches only its own specific amino acid to the correct tRNA, using ATP B1 the ribosome itself only checks that the anticodon of an incoming tRNA is complementary to the codon in the A site; it cannot check which amino acid that tRNA is carrying B1 because each tRNA is reliably loaded with the one amino acid that its anticodon corresponds to, matching a codon to its correct anticodon is what actually determines which amino acid is added, so the specificity of the loading enzymes is what makes the whole system accurate

Question 12

Electron micrographs of active cells often show several ribosomes attached to and spaced along the same mRNA molecule, an arrangement called a polysome. Suggest what advantage this gives the cell.

Accepted Answer

B1 each ribosome in the polysome is independently translating the same mRNA molecule, at a different position along it B1 several copies of the same polypeptide are therefore being synthesised from the one transcript at the same time, rather than one ribosome finishing before the next begins B1 this lets the cell produce many copies of a protein rapidly from a single mRNA molecule, without needing to transcribe further copies of the mRNA to achieve the same output

Question 13

The template strand of a short gene, written 3′ to 5′, reads TAC ACG TTC CGA ATC. Use the codons AUG = methionine, UGC = cysteine, AAG = lysine, GCU = alanine, UAG = stop. Give the mRNA sequence transcribed from this template, give the anticodon of the tRNA that delivers lysine, and give the number of amino acids in the finished polypeptide.

Accepted Answer

B1 the mRNA is complementary to the template strand, with U wherever the template has A: TAC gives AUG, ACG gives UGC, TTC gives AAG, CGA gives GCU and ATC gives UAG, so the mRNA is AUG UGC AAG GCU UAG B1 the anticodon is complementary to the codon rather than identical to it, so the anticodon that delivers lysine, whose codon is AAG, is UUC A1 there are five codons, but UAG is a stop codon and specifies no amino acid, so the polypeptide is four amino acids long

Question 14

During translation, the P site of a ribosome holds the tRNA carrying the growing polypeptide chain, and the A site holds the tRNA bringing the next amino acid. Name the bond formed between the chain and that next amino acid, and name the type of reaction that forms it.

Accepted Answer

B1 a peptide bond B1 a condensation reaction, which releases a molecule of water

Question 15

State the sedimentation coefficient of a eukaryotic ribosome, and state the sedimentation coefficient of a ribosome found in a bacterium.

Accepted Answer

B1 a eukaryotic ribosome is 80S B1 a ribosome in a bacterium is 70S