HSC Exam Practice

Sample response. Translation is the process by which a ribosome reads the codon sequence of an mRNA molecule and uses tRNAs to assemble a polypeptide whose amino acid order matches the codon order.

Marking notes. 1 mark for identifying translation as decoding of mRNA into a polypeptide; 1 mark for naming at least one machinery component (ribosome or tRNA) involved in that decoding.

Sample response. A codon is a three-base sequence on mRNA that specifies one amino acid. An anticodon is a three-base sequence on tRNA that is complementary to a codon and pairs with it during translation. Codons are read by the ribosome; anticodons deliver the matching amino acid.

Marking notes. 1 mark for correctly defining codon (mRNA, three bases, specifies amino acid); 1 mark for correctly defining anticodon (tRNA, three bases, complementary to codon); 1 mark for identifying the parent molecule of each (mRNA vs tRNA).

Sample response. The ribosome binds the mRNA in the cytoplasm and reads its codons three bases at a time, holding successive tRNAs in position so their anticodons can pair with the codons being read, and catalysing peptide bond formation between adjacent amino acids so the polypeptide elongates.

Marking notes. 1 mark for stating the ribosome reads mRNA codons / coordinates tRNA pairing; 1 mark for stating it catalyses or enables peptide bond formation / polypeptide assembly.

Sample response. AUG → anticodon UAC. GAA → anticodon CUU. CCU → anticodon GGA.

Marking notes. 1 mark per correct anticodon. Penalise the use of T in place of U (RNA contains uracil, not thymine).

Sample response. At the ribosome, a tRNA anticodon pairs with a complementary mRNA codon, bringing one specific amino acid into position. A second tRNA pairs with the next codon, delivering the next amino acid adjacent to the first. The ribosome then catalyses formation of a peptide bond linking the two amino acids. The ribosome translocates one codon along the mRNA and the cycle repeats — each successful pairing and peptide bond adds another amino acid, so the polypeptide elongates one residue at a time.

Marking notes. 1 mark for codon-anticodon pairing bringing the correct amino acid; 1 mark for two adjacent amino acids held in position at the ribosome; 1 mark for ribosome catalysing peptide bond formation; 1 mark for explicit repetition / cycle producing elongation.

Sample response. Translation occurs in the cytoplasm at ribosomes (free in the cytoplasm or bound to the rough endoplasmic reticulum for secreted proteins). mRNA is produced in the nucleus by transcription because that is where the DNA template is located, then is exported through nuclear pores into the cytoplasm where translation can occur.

Marking notes. 1 mark for correct location of translation (cytoplasm / ribosomes); 1 mark for explaining the location difference in terms of DNA being in the nucleus and mRNA being exported.

Sample response (a). Mean released polypeptide length falls steeply as puromycin concentration rises, then plateaus near zero. With no puromycin, polypeptides average ≈ 360 amino acids; by 20 µM the mean is already roughly halved (≈ 180 aa), and by 100 µM the mean is only ≈ 22 aa. The relationship is negative and non-linear, with most of the change occurring between 0 µM and 40 µM.

Sample response (b). Translation depends on the ribosome catalysing a peptide bond between the amino acid attached to one tRNA and the growing chain on the previous tRNA, then translocating to the next codon. Puromycin mimics a charged tRNA and enters the ribosome's A-site, so the ribosome forms a peptide bond between the growing polypeptide and puromycin. Because puromycin lacks the structure needed for further elongation, no further codon-anticodon pairing or peptide bond formation can occur and the truncated polypeptide is released. At higher puromycin concentrations, puromycin enters the A-site earlier during translation, so chains are released after fewer codons have been read — exactly the pattern shown by the falling curve.

Marking notes. Part (a) — 1 mark for stating the negative / decreasing relationship; 1 mark for quoting at least one supporting figure from the data (e.g. 360 → 22). Part (b) — 1 mark for identifying that puromycin enters the ribosome and accepts a peptide bond; 1 mark for explaining that this terminates further codon-anticodon pairing / elongation; 1 mark for linking higher puromycin concentration to earlier termination and therefore shorter polypeptides.

Sample response. The statement is correct. Translation is the process in which a ribosome reads an mRNA codon sequence three bases at a time, while tRNAs deliver specific amino acids using complementary anticodons; the ribosome then catalyses peptide bonds between adjacent amino acids to elongate the polypeptide. Because each codon specifies one amino acid, the order of codons in the mRNA directly determines the order of amino acids in the polypeptide, and therefore the primary structure of the protein. Insulin is a clear example: it is synthesised as preproinsulin from the INS mRNA, and its mature two-chain hormone form depends on a specific sequence of amino acids that allows correct folding and the formation of three disulfide bridges between cysteine residues. A single codon change can be enough to break this. In β-globin, a single GAG → GUG mutation substitutes valine for glutamic acid at residue 6, producing HbS that polymerises under low oxygen and causes sickle-cell disease — even though the ribosome and tRNAs themselves are functioning normally and faithfully translating the new codon. The lesson's misconceptions box reinforces that the ribosome does not "fix" wrong codons; it simply matches whichever codon is presented to its complementary anticodon. The statement is therefore correct: translation has to be accurate at every codon for the resulting polypeptide to fold into a functional protein. However, accuracy is necessary but not sufficient — correct translation of an already-mutated mRNA, as in sickle-cell anaemia, still produces a non-functional protein, so the statement should be read as "correct translation of a correct codon sequence" rather than as a guarantee that translation alone determines function.

Marking notes. 1 mark — defines translation and identifies roles of mRNA, tRNA and ribosome. 1 mark — explains codon-anticodon pairing as the link between codon order and amino acid order. 1 mark — explains peptide bond formation and polypeptide elongation. 1 mark — uses at least one named protein example (insulin or β-globin) and links its function to its amino acid sequence. 1 mark — describes consequences of a single codon change (e.g. Glu → Val in β-globin producing HbS) on protein function. 1 mark — addresses the misconception that the ribosome "corrects" wrong codons. 1 mark — reaches an explicit evaluative judgement that qualifies "essential" correctly (accurate translation is necessary but not sufficient if the input mRNA is itself mutated). Cap at 5 if no named protein example, or if codon-anticodon pairing is not explicitly invoked.