HSC Exam Practice

Sample response. Recombinant DNA is DNA formed by combining genetic material from two or more different sources, typically by cutting a chosen DNA fragment from donor DNA and joining it into a vector using restriction enzymes and DNA ligase.

Marking notes. 1 mark for identifying recombinant DNA as DNA combining genetic material from different sources; 1 mark for referencing the cutting + joining of DNA fragments (or naming restriction enzymes and ligase / vector).

Sample response. A restriction enzyme cuts double-stranded DNA at a specific recognition sequence, producing fragments with characteristic (often sticky) ends. DNA ligase, in contrast, joins DNA fragments together by sealing the sugar–phosphate backbone of the two strands, so a donor fragment and an opened vector can be combined to form one continuous recombinant DNA molecule. The two enzymes therefore perform opposite and complementary actions — cutting versus joining — and both are required to make recombinant DNA.

Marking notes. 1 mark for restriction-enzyme role (cuts DNA at specific recognition sequences); 1 mark for ligase role (joins DNA fragments / seals sugar–phosphate backbone); 1 mark for explicit contrast that the two enzymes do opposite, complementary jobs (cutting vs joining) and both are required.

Sample response. A vector (commonly a plasmid) is a carrier that transports the inserted DNA into a host cell. A host cell receives the recombinant DNA and provides the molecular machinery to either replicate the inserted DNA (as the cell divides) or express it as a functional protein.

Marking notes. 1 mark for vector = carrier of inserted DNA into a host cell; 1 mark for host = cell that receives recombinant DNA and replicates or expresses it.

Sample response. A transgenic organism contains DNA that has been inserted from another source, often a different species, using recombinant DNA technology (restriction enzymes, ligase, a vector, and a host cell). A selectively bred organism is produced by choosing parents that already carry desirable traits and allowing reproduction to reshuffle the existing alleles. The two are fundamentally different because selective breeding can only recombine alleles already present in a breeding population, while recombinant DNA technology can introduce DNA that does not exist in that population at all (for example, a bacterial Bt-toxin gene in a cotton plant).

Marking notes. 1 mark for transgenic = inserted DNA from another source, produced using recombinant DNA methods. 1 mark for selectively bred = parents chosen for existing traits; alleles reshuffled through reproduction. 1 mark for explicitly contrasting inserted DNA versus reshuffled existing alleles, ideally with a named example.

Sample response. A vector is required to carry the recombinant DNA into a host cell, because naked DNA usually cannot enter and be maintained inside a cell on its own. A host cell is required because the recombinant DNA cannot be replicated or expressed outside a living cell — the host provides the polymerases, ribosomes and energy supply needed to copy the DNA or translate it into a protein. Neither alone is sufficient: vector with no host has no machinery to act on the DNA; host with no vector has nothing to receive.

Marking notes. 1 mark for vector necessary (carrier into a host cell); 1 mark for host necessary (provides replication / transcription / translation machinery). Award both only if the response makes clear neither alone is sufficient.

Sample response (a). Insecticide active ingredient applied to Australian cotton fell sharply between 1992 and 2018. From a baseline of about 7.0 kg/ha in 1992 (pre-Bt), use fell to about 4.5 kg/ha in 2000, then to 1.6 kg/ha in 2008, and to only ~0.6 kg/ha by 2018 — a reduction of roughly 90% over the period.

Sample response (b). The Bt gene, originally from the soil bacterium Bacillus thuringiensis, is the donor DNA. Restriction enzymes cut the Bt gene out of bacterial DNA and also cut a vector (commonly a plasmid carried in Agrobacterium tumefaciens) at compatible sites, producing complementary sticky ends. DNA ligase joins the Bt gene into the plasmid, producing recombinant DNA. The recombinant plasmid is used to transfer the Bt gene into cotton cells (the host cells), which are then regenerated into whole transgenic cotton plants that express the Bt protein in their tissues.

Sample response (c). Bt cotton plants continuously produce Bt toxin in their own tissues, which kills Helicoverpa caterpillars (the main cotton pest) when they feed on the plant. Because the plant itself controls the pest, growers no longer need to apply repeated chemical insecticide sprays, which is why the active ingredient per hectare has fallen from ~7.0 kg/ha (1992) to ~0.6 kg/ha (2018). The transgenic trait substitutes for chemical control, accounting for the magnitude of the reduction.

Marking notes. Part (a) — 1 mark for identifying a clear downward trend; 1 mark for quoting at least one figure (any pair, e.g. 7.0 → 0.6 kg/ha, or 90% reduction). Part (b) — 1 mark for identifying the Bt gene from Bacillus thuringiensis as donor DNA; 1 mark for naming a vector (plasmid / Agrobacterium) and the cutting + joining steps (restriction enzyme + ligase); 1 mark for naming cotton cells as the host cells regenerated into transgenic cotton plants. Part (c) — 1 mark for linking Bt-toxin expression in the plant to reduced need for sprays, explicitly connecting transgenic trait to the data trend.

Sample response. Recombinant DNA technology is highly useful in both agriculture and medicine because it allows DNA to be inserted directly into cells rather than relying on alleles already present in a breeding population. The same four-step toolchain — cut donor DNA and vector with a restriction enzyme, join the fragment into the vector with DNA ligase, insert the recombinant vector into a host cell, then use the host to replicate or express the gene — supports very different applications depending on the donor DNA and host chosen. In medicine, recombinant human insulin ("Humulin") is produced by inserting the human insulin gene into a plasmid, transferring the plasmid into Escherichia coli, and growing the transgenic bacteria in fermenters; the bacteria express human insulin, which is purified for diabetic patients. This was not achievable by selective breeding: there is no breedable population of organisms that already produces human insulin, so reshuffling existing alleles cannot generate the human protein, while inserting the human gene into a bacterium can. In agriculture, Australian Bt cotton was produced by inserting the Bt toxin gene from Bacillus thuringiensis into cotton cells via an Agrobacterium plasmid vector, producing transgenic cotton plants that synthesise Bt toxin in their tissues and resist Helicoverpa caterpillars; commercial adoption has reduced insecticide active ingredient applied per hectare from ~7.0 kg/ha in 1992 to ~0.6 kg/ha by 2018. Again, this trait could not be generated by selective breeding because no allele for Bt-toxin production exists in cotton's gene pool. Both examples demonstrate the lesson's central point: recombinant DNA technology is useful precisely because it adds capabilities that selective breeding cannot reach — importing genes across species, producing target proteins at industrial scale, and inserting traits not present in the recipient species' gene pool. Its usefulness is not unconditional (regulatory approval, monitoring of unintended ecological effects, and ethical scrutiny are all required), but the technology has produced measurable, transformative benefits in both fields, and on the lesson's definition it is correctly judged as a highly useful, methodologically distinct tool rather than a "more complicated form of selective breeding".

Marking notes. 1 mark — Identifies recombinant DNA technology as a stepwise toolchain (cut → join → insert → use) rather than a single event. 1 mark — Names at least one valid medical example (recombinant insulin in E. coli, recombinant human growth hormone, recombinant clotting factors, vaccine production) and maps it onto the toolchain. 1 mark — Names at least one valid agricultural example (Bt cotton, Golden Rice, herbicide-tolerant soybean) and maps it onto the toolchain. 1 mark — Identifies a usefulness criterion supported by the chosen example (e.g. industrial-scale production of a human protein, reduced insecticide use, vitamin-A enrichment of staple food, decoupling supply from livestock). 1 mark — Explicitly contrasts recombinant DNA technology with selective breeding using the lesson's framing (inserted DNA across species vs reshuffling existing alleles within a breeding pool). 1 mark — Recognises a limitation, qualification, or ethical / regulatory caveat on usefulness (e.g. ecological effects, regulatory approval, public concern, monogenic-trait scope). 1 mark — Reaches an explicit evaluative judgement on usefulness, using precise lesson terminology (recombinant DNA, restriction enzyme, ligase, vector, host cell, transgenic).