HSC Exam Practice

Sample response. Transcription is the process by which an mRNA copy is produced from a DNA template strand, with RNA nucleotides pairing complementarily with the exposed bases of the template.

Marking notes. 1 mark for identifying transcription as the production of mRNA from DNA; 1 mark for naming the DNA template strand and/or the role of complementary pairing.

Sample response. On mRNA, adenine on the DNA template pairs with uracil. This differs from DNA, where adenine pairs with thymine. RNA uses uracil instead of thymine.

Marking notes. 1 mark for identifying uracil as the mRNA pairing base; 1 mark for the contrast with thymine in DNA.

Sample response. The DNA template strand acts as the read-off pattern during transcription. Its exposed bases determine, by complementary base pairing, which RNA nucleotides are joined to form the mRNA, so the template fixes the mRNA's base sequence.

Marking notes. 1 mark for identifying it as the strand used to build mRNA; 1 mark for explaining that complementary pairing of RNA nucleotides to the template determines the mRNA sequence.

Sample response. Template (3'→5'): T A C G G A C C T A T G.
mRNA (5'→3'): A U G C C U G G A U A C.

Marking notes. 1 mark for correct A↔U and T↔A pairing; 1 mark for correct C↔G pairing throughout; 1 mark for correctly grouping into four codons in 5'→3' orientation. Accept the sequence without 5'/3' labels if codons are correct.

Sample response. In eukaryotic cells, DNA remains in the nucleus and the next stage of polypeptide synthesis takes place in the cytoplasm at the ribosome. The cell therefore needs a temporary copy of the gene that can carry the coded information away from the DNA. mRNA performs this role by holding the copied sequence — including its codons — in transferable form. This protects the original DNA from being moved around and allows multiple mRNA copies to be made from the same gene when many copies of a protein are required.

Marking notes. 1 mark — DNA stays in the nucleus in eukaryotic cells. 1 mark — cell needs a portable copy of the gene's information for use outside the nucleus. 1 mark — mRNA fulfils this role / carries the codon sequence. 1 mark — additional advantage (protects DNA, or many mRNA copies from one gene, or allows simultaneous protein synthesis).

Sample response. A codon is a sequence of three consecutive bases on an mRNA molecule that carries transferable coded information. Codons appear along the mRNA strand after transcription is complete.

Marking notes. 1 mark for "three-base sequence carrying coded information"; 1 mark for locating codons on mRNA (not DNA, not on the ribosome).

Sample response (a). CFTR mRNA level differs greatly across the four tissues. Bronchial epithelium has the highest level (100 relative units), followed by pancreatic duct (83), sweat gland (51) and skeletal muscle (15) — a difference of roughly 6–7× between the highest and lowest tissue. Tissues with epithelial / secretory function carry far more CFTR mRNA than skeletal muscle.

Sample response (b). Every cell carries the same CFTR DNA sequence, so the difference cannot be in the DNA copy number — it must arise during transcription. In tissues that need CFTR protein (e.g. bronchial epithelium and pancreatic duct), the CFTR gene is transcribed at a high rate, producing many mRNA copies per cell. In skeletal muscle, CFTR is rarely transcribed because the cell does not require its ion-channel function, so very few mRNA molecules are produced. Because mRNA is a temporary copy of the gene, cells can match the number of transcripts they make to their current protein needs without changing the underlying DNA.

Marking notes. Part (a): 1 mark for ordering the four tissues from highest to lowest; 1 mark for quoting at least one supporting figure. Part (b): 1 mark for identifying that DNA copy number is constant across tissues; 1 mark for attributing the difference to differential transcription rate; 1 mark for connecting transcription rate to cell function / protein need (epithelial vs muscle).

Sample response. The statement is valid. Transcription is the process of producing an mRNA copy from a DNA template strand, with RNA nucleotides pairing complementarily with the exposed bases of the template (A–U, T–A, C–G, G–C). Because the rule is base-by-base, the mRNA sequence produced is entirely determined by the DNA base order of the template strand. If the CFTR DNA sequence changes — for example by a substitution or a deletion such as ΔF508 — the template strand presents altered bases to the RNA polymerase, so the mRNA built by complementary pairing will carry different RNA bases at those positions. Because the mRNA is read in three-base codons, any altered base sits inside a codon, so the message the mRNA carries away from the nucleus is already different before translation even begins. This means the effect of the DNA change is not delayed until protein synthesis: the very act of transcription copies the change into the mRNA. The statement is therefore biologically defensible — transcription depends directly on the DNA base order, and a sequence change in the CFTR DNA can alter the codons carried by the mRNA at the transcription step itself, before translation occurs.

Marking notes. 1 mark — defines transcription and names the template strand. 1 mark — describes complementary base pairing rules including A–U substitution. 1 mark — explains that the mRNA sequence is determined by the template base order. 1 mark — applies the logic to a DNA sequence change in CFTR producing an altered mRNA. 1 mark — explicitly links altered base to altered codon information. 1 mark — reaches a justified evaluative judgement that the statement is valid because the change is copied at transcription, before translation begins. Cap at 5 if the response does not include codons explicitly.