HSC Exam Practice

Sample response. A DNA nucleotide consists of (i) a sugar (deoxyribose), (ii) a phosphate group and (iii) a nitrogenous base (adenine, thymine, cytosine or guanine).

Marking notes. 1 mark for naming the three component types (sugar, phosphate, nitrogenous base); 1 mark for any additional precision (e.g. naming the sugar as deoxyribose or listing valid bases). Naming "amino acid" instead of "nitrogenous base" scores 0 for that component.

Sample response. The Watson and Crick model describes DNA as a double helix made of two strands of nucleotides. The two strands are held together by hydrogen bonds between paired bases. The pairing is complementary: adenine pairs with thymine (A–T) and cytosine pairs with guanine (C–G).

Marking notes. 1 mark for double-helix shape with two strands; 1 mark for hydrogen bonds between paired bases; 1 mark for correct complementary pairing rules (A–T and C–G).

Sample response. Original strand 5'-A T G C C A T-3' → complementary strand 3'-T A C G G T A-5'.

Marking notes. 2 marks for the fully correct complementary sequence. 1 mark for a sequence that uses the correct pairing rules but contains a single base error or is written in the wrong direction. 0 marks if uracil is included (DNA uses thymine, not uracil) or if the wrong pairing rules are applied.

Sample response. Semiconservative replication is the form of DNA replication in which the two original strands separate, and each acts as a template for a new complementary strand, so that every daughter DNA molecule contains one original (template) strand and one newly synthesised strand.

Marking notes. 1 mark for identifying that each daughter molecule contains one original strand and one new strand; 1 mark for referring to the original strand acting as a template (or for describing strand separation followed by complementary synthesis).

Sample response. Complementary base pairing means adenine always pairs with thymine and cytosine always pairs with guanine. During replication the two original strands separate, exposing the bases on each strand. Free nucleotides are added to each exposed base according to these pairing rules, so each new strand has a sequence determined exactly by the original strand it copies — the original sequence guides the new one, rather than bases being added randomly.

Marking notes. 1 mark for stating the complementary pairing rules (A–T, C–G); 1 mark for describing strand separation and use of each original as a template; 1 mark for explicitly linking complementary pairing to accuracy (the original strand specifies which base must be added at each position).

Sample response. Accurate replication preserves the hereditary information stored in the DNA sequence, so daughter cells — and ultimately offspring — inherit the correct instructions for proteins and cell function. This reliable inheritance is what allows the species' characteristics to persist across generations, supporting continuity of species.

Marking notes. 1 mark for identifying that accurate replication preserves hereditary information / the DNA sequence; 1 mark for linking this preservation to inheritance across generations (continuity of species).

Sample response (a). Between generation 0 and generation 1, the percentage of heavy DNA falls from 100% to 0%, while the percentage of hybrid DNA rises from 0% to 100%. After one round of replication, all DNA is in the hybrid band; no heavy DNA remains and no light DNA has yet appeared.

Sample response (b). At generation 1, every DNA molecule is a hybrid because each parental ¹⁵N strand has been used as a template and paired with a newly synthesised ¹⁴N strand. At generation 2, these hybrid molecules separate again; each ¹⁵N strand templates another new ¹⁴N strand (producing another hybrid molecule), but each ¹⁴N strand from generation 1 also templates a new ¹⁴N strand, producing a fully light (¹⁴N/¹⁴N) molecule. Half the molecules are therefore hybrid and half are light, exactly as the graph shows.

Marking notes. Part (a): 1 mark for stating heavy 100% → 0%; 1 mark for hybrid 0% → 100% (numerical change required). Part (b): 1 mark for identifying that hybrid molecules contain one original ¹⁵N strand and one new ¹⁴N strand; 1 mark for explaining that at gen 2, the ¹⁴N strand of a hybrid templates an entirely ¹⁴N/¹⁴N (light) molecule; 1 mark for explicitly linking the 50/50 hybrid + light split at gen 2 to the semiconservative model.

Sample response (a). Within each species, %A is approximately equal to %T, and %C is approximately equal to %G. For example, in human DNA A = 30.4% and T = 30.1%, while C = 19.6% and G = 19.9%. Small differences are due to measurement error.

Sample response (b). The Watson and Crick model proposes that DNA is double-stranded with complementary base pairing — adenine on one strand always pairs with thymine on the other, and cytosine with guanine. For every A on one strand there must be a T opposite on the other strand, so when both strands are counted together the total %A is forced to equal %T, and similarly %C must equal %G across the whole molecule.

Marking notes. Part (a): 1 mark for identifying the %A = %T / %C = %G relationship; 1 mark for quoting at least one supporting figure from the table. Part (b): 1 mark for stating that A pairs with T and C pairs with G in the double helix; 1 mark for the mechanistic link (every A on one strand requires a T opposite, so the counts must be equal across the whole molecule).

Sample response. The reliability of DNA replication is overwhelmingly a consequence of DNA structure rather than chance. The Watson and Crick model describes DNA as a double helix of two strands of nucleotides, in which adenine on one strand always pairs with thymine on the other and cytosine always pairs with guanine, the pairs held together by hydrogen bonds. This structural arrangement is what makes accurate copying possible. During semiconservative replication the two original strands separate, and each acts as a template: free nucleotides are added one by one according to the complementary pairing rules, so the sequence of the new strand is specified by the sequence of the original. The result is two daughter DNA molecules, each containing one original strand and one newly synthesised strand — built to the same pattern as the parent rather than from scratch. If replication were not anchored to existing strands in this way, bases would be added randomly and the original sequence would not be preserved. The biological importance of this structural reliability is shown by what happens when replication errors do occur: even a single incorrect base can change the DNA sequence, potentially altering later protein production and cell behaviour, and if such an error is passed on it can affect daughter cells and future generations. Errors are minimised precisely because each new strand is matched against an existing template; chance plays a role only in the rare cases where the pairing rules are not perfectly followed. Therefore the claim is supported: the molecular structure of DNA — double-stranded, complementary, antiparallel — is the mechanism that makes replication reliable, and the limited consequences of the occasional error confirm rather than undermine this conclusion.

Marking notes. 1 mark — outlines the Watson and Crick model (double helix, nucleotides with sugar + phosphate + base, complementary pairing A–T and C–G, hydrogen bonds). 1 mark — identifies semiconservative replication as: strands separate, each old strand acts as a template, each daughter molecule has one old + one new strand. 1 mark — explicitly explains how complementary pairing forces the new strand sequence to match the original (mechanism of accuracy). 1 mark — contrasts this with a hypothetical "from-scratch" or random model, showing why structure (not chance) is doing the work. 1 mark — discusses replication errors: even a single-base change alters the sequence and can affect protein production / cell behaviour. 1 mark — links replication errors to inheritance (errors passed to daughter cells / future generations) and uses this to support — not contradict — the structural reliability argument. 1 mark — reaches an explicit, evidence-based judgement that DNA structure is the primary mechanism of replication reliability, using lesson terminology (template, semiconservative, complementary base pairing, hereditary information).