Inquiry Question 3: How does genetic information flow from DNA to functional proteins?
Construct appropriate representations to model and compare the processes of transcription and translation, including but not limited to: the structure of DNA and the contributions of Watson, Crick, Franklin and Wilkins
A focused answer to the HSC Biology Module 5 dot point on DNA structure. The double helix, the sugar-phosphate backbone, the four bases and the A-T/G-C base pairing rules, the historical contributions of Watson, Crick, Franklin (Photograph 51) and Wilkins, the forms DNA takes in eukaryotes vs prokaryotes, and worked HSC past exam questions.
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What this dot point is asking
NESA wants you to describe the molecular structure of DNA AND attribute the discovery accurately, including Rosalind Franklin and Maurice Wilkins as well as the more famous Watson and Crick. The historical attribution is important: NESA has explicitly rewarded responses that name Franklin's contribution since the 2017 syllabus update.
The answer
Structure of DNA
DNA (deoxyribonucleic acid) is a double helix of two antiparallel polynucleotide strands.
Each strand has three components per nucleotide:
- A deoxyribose sugar (a 5-carbon sugar).
- A phosphate group attached to the 5' carbon of the sugar.
- One of four nitrogenous bases attached to the 1' carbon.
The sugar and phosphate of one nucleotide bond to the sugar and phosphate of the next, forming a sugar-phosphate backbone on the outside of the helix. The bases project inward and pair with bases from the opposite strand.
The four bases and complementary base pairing
The four bases are:
- Adenine (A) and guanine (G) are purines (two rings, larger).
- Thymine (T) and cytosine (C) are pyrimidines (one ring, smaller).
The two strands are held together by hydrogen bonds between complementary base pairs:
- A pairs with T by two hydrogen bonds.
- G pairs with C by three hydrogen bonds.
This pairing rule (Chargaff's rule) means the amount of A in DNA always equals the amount of T, and G equals C.
Antiparallel strands
The two strands run in opposite directions. One runs 5' to 3'; the other runs 3' to 5'. This antiparallel orientation matters for DNA replication (DNA polymerase only synthesises in the 5' to 3' direction, which produces the leading vs lagging strand distinction).
The double helix
The two strands twist around each other in a right-handed double helix. One full turn is roughly 3.4 nm and contains about 10 base pairs. The diameter is about 2 nm.
Key contributions to discovery
- Rosalind Franklin (King's College London)
- A skilled X-ray crystallographer. In 1952 she produced Photograph 51, an X-ray diffraction image of DNA that clearly showed the helical structure and the regular spacing of the bases. She was on the verge of publishing her own model.
- Maurice Wilkins (King's College London)
- Franklin's colleague. Showed Photograph 51 to Watson without Franklin's knowledge or permission. Wilkins shared the 1962 Nobel Prize with Watson and Crick; Franklin had died in 1958 and was not eligible.
- James Watson and Francis Crick (Cambridge)
- Built the first accurate physical model of the DNA double helix in 1953, using Franklin's X-ray data plus Erwin Chargaff's chemical analysis showing and . Their paper "Molecular Structure of Nucleic Acids" was published in Nature on 25 April 1953. It is one of the most cited papers in biology.
- Erwin Chargaff (Columbia)
- Showed in the 1940s that in any DNA sample, the amount of A equals the amount of T, and G equals C. These Chargaff's rules were the crucial constraint Watson and Crick used to figure out base pairing.
Why the historical attribution matters
For many decades, the Watson-Crick attribution dominated public understanding, while Franklin's role was understated. The current HSC syllabus explicitly asks students to recognise Franklin's contribution. Top-band answers name her by name and identify Photograph 51 as the key piece of evidence.
The forms in which DNA exists: eukaryotes vs prokaryotes
The same chemical molecule (the double helix described above) is packaged and stored differently depending on the cell type. NESA asks you to compare these forms directly.
In eukaryotes (animals, plants, fungi, protists), DNA is:
- Linear - organised into several separate chromosomes (humans have 46).
- Wound around histone proteins - the DNA coils tightly around bead-like histones to form chromatin, which condenses further into visible chromosomes during division.
- Enclosed in a nucleus - separated from the cytoplasm by the nuclear envelope.
- Present in large amounts (many genes). A small amount of separate circular DNA also exists in mitochondria and chloroplasts.
In prokaryotes (bacteria, archaea), DNA is:
- Circular - usually a single circular chromosome.
- "Naked" - not wound around histones (it is supercoiled and associated with some packaging proteins, but lacks the histone-based chromatin of eukaryotes).
- Free in the cytoplasm - concentrated in a region called the nucleoid, with no surrounding nucleus.
- Often accompanied by plasmids - small, separate circular DNA molecules carrying a few extra genes (for example, antibiotic resistance).
Examples in context
Example 1. Why G-C content matters in PCR primer design. When NSW Health pathology labs design a PCR primer to detect a pathogen such as Mycobacterium tuberculosis, they aim for primers that are 40-60 percent G-C. The reason traces directly back to Watson and Crick's structure: every G-C pair is held by three hydrogen bonds, while every A-T pair is held by only two. A high G-C primer therefore needs a higher annealing temperature to stay bound to the template, which gives the PCR more specificity and fewer false positives. A primer that is too A-T rich melts off at the working temperature and amplifies the wrong region.
Example 2. Franklin, Photograph 51, and the King's College controversy. In May 1952, Rosalind Franklin and her PhD student Raymond Gosling exposed a wet fibre of DNA to an X-ray beam for over 60 hours, producing Photograph 51. The image showed an unmistakable X-shaped diffraction pattern, the signature of a helix, with precise spacing that revealed a 3.4 nm pitch and 10 bases per turn. In January 1953, Maurice Wilkins showed the unpublished photograph to James Watson at Cambridge without Franklin's knowledge. Within weeks, Watson and Crick had built their model. Franklin died of ovarian cancer in 1958, four years before Watson, Crick and Wilkins received the Nobel Prize.
(The practice items that previously sat in a "Try this" box - the base-percentage calculation, the justification of Franklin's inclusion, and the strand-separation temperature comparison - are now in the graded practice questions for this page, each with a full marking solution.)
Exam-style practice questions
Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2022 HSC3 marksOutline the ways in which the DNA of prokaryotes and eukaryotes differ.Show worked answer →
Full marks require outlining (not just identifying) differences in prokaryotic vs eukaryotic DNA. The guidelines award 3 marks for outlining the differences, 2 for outlining one difference (or identifying differences), 1 for some relevant information.
Sample answer points (from the marking guidelines):
- Prokaryotic DNA: a circular molecule, found free in the cytoplasm, carries a small number of genes, and is not tightly coiled around histone proteins.
- Eukaryotic DNA: a linear molecule, found in the nucleus, carries a large number of genes, and is tightly coiled around histone (and other) proteins.
Keep the answer focused on DNA/chromosome structure and location, not on general cell differences.
Source: NESA 2022 HSC Biology examination and marking guidelines.
Practice questions
Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.
foundation2 marksName the three components of a single DNA nucleotide and state which component varies between nucleotides.Show worked solution →
1 mark - the three components. A deoxyribose sugar (5-carbon sugar), a phosphate group, and a nitrogenous base.
1 mark - the variable part. Only the nitrogenous base varies (adenine, thymine, guanine or cytosine); the sugar and phosphate are identical in every nucleotide.
Naming "sugar, phosphate, base" earns the first mark; the second mark is specifically for identifying the base as the variable component, which is the point examiners are testing.
foundation3 marksState the complementary base-pairing rules of DNA, and for each pair give the number of hydrogen bonds that hold it together.Show worked solution →
- 1 mark - the pairing rule
- Adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). A purine always pairs with a pyrimidine, keeping the helix at a constant diameter.
- 1 mark - A-T bonds
- A-T is held by two hydrogen bonds.
- 1 mark - G-C bonds
- G-C is held by three hydrogen bonds.
A common slip is to pair A with G or T with C; only the exact A-T / G-C pairing earns the first mark. The bond counts (2 and 3) must be the right way round.
foundation3 marksA double-stranded DNA sample contains 18 percent guanine. Calculate the percentage of adenine, thymine and cytosine in the sample, showing your reasoning.Show worked solution →
- 1 mark - cytosine
- By Chargaff's rule , so cytosine .
- 1 mark - reasoning for A and T
- The four bases total . , so . Because , this 64 percent is split evenly.
- 1 mark - adenine and thymine
- and .
Full marks require the working (showing and ), not just the final numbers. A frequent error is to assume all four bases are 25 percent.
core4 marksExplain how the structure of the DNA molecule allows the two strands to be held together yet separated when needed, referring to base pairing, hydrogen bonds and antiparallel orientation.Show worked solution →
Award up to 4 marks for an explanation that links structural features to the molecule being both stable and separable.
- Base pairing (1 mark)
- The bases project inward and pair complementarily (A with T, G with C), so the two strands hold a precise, matched sequence.
- Hydrogen bonds (1 mark)
- Each pair is joined by hydrogen bonds (two for A-T, three for G-C). Individually these are weak, so the strands can be unzipped by enzymes; collectively, across millions of pairs, they hold the helix firmly together.
- Antiparallel orientation (1 mark)
- The two strands run in opposite directions (5' to 3' against 3' to 5'), which allows complementary bases to align correctly along the whole length.
- Link to function (1 mark)
- Because the bonds are weak and the sequence is complementary, each separated strand acts as a template for accurate copying (replication) or transcription. An answer that lists features without linking them to "held together yet separable" caps at 3 marks.
core4 marksDescribe the contributions of Franklin, Wilkins, Watson and Crick to the discovery of the structure of DNA.Show worked solution →
Award 1 mark for each contribution correctly attributed (up to 4 marks).
- Rosalind Franklin (1 mark)
- An X-ray crystallographer who produced Photograph 51 (1952), the X-ray diffraction image that revealed DNA's helical shape, its dimensions and the regular spacing of the bases.
- Maurice Wilkins (1 mark)
- Franklin's colleague at King's College London who showed Photograph 51 to Watson (without Franklin's knowledge); he had also worked on X-ray studies of DNA.
- James Watson and Francis Crick (1 mark)
- At Cambridge, they built the first accurate physical model of the double helix (1953), interpreting Franklin's diffraction data and Chargaff's rules.
- Synthesis / accuracy (1 mark)
- A response earns the final mark for correctly distinguishing the roles - data and evidence (Franklin, Wilkins) versus model-building and interpretation (Watson, Crick) - rather than blurring them. Naming only Watson and Crick caps the answer at 2 marks.
core3 marksOutline the ways in which the form of DNA in a prokaryotic cell differs from the form of DNA in a eukaryotic cell.Show worked solution →
- 1 mark - shape and location
- Prokaryotic DNA is a single circular molecule found free in the cytoplasm (in the nucleoid region); eukaryotic DNA is linear, organised into several chromosomes, and enclosed within a nucleus.
- 1 mark - packaging
- Eukaryotic DNA is wound around histone proteins to form chromatin; prokaryotic DNA is largely "naked" (not bound to histones, though it is supercoiled).
- 1 mark - extra DNA / gene number
- Prokaryotes also carry small circular plasmids; eukaryotes carry a much larger amount of DNA / more genes.
The command word is "outline", so brief points are sufficient, but each contrast must name the feature for both cell types to earn the mark.
exam6 marksTwo DNA samples are heated until their strands separate. Sample X separates at 95 degrees C; sample Y separates at 88 degrees C. (a) Account for the difference in separation temperatures in terms of DNA structure. (b) Deduce which sample has the higher G-C content and justify your deduction. (c) Explain why this relationship between base composition and separation temperature is a direct consequence of the Watson-Crick model.Show worked solution →
Target a sequenced response that links base composition, hydrogen bonding and the energy needed to separate the strands.
- (a) Account for the difference (1-2 marks)
- The two strands are held together by hydrogen bonds between base pairs. Separating the strands ("melting") requires enough thermal energy to break these bonds. Sample X separates at a higher temperature, so more energy is needed, meaning sample X has more hydrogen bonds per unit length.
- (b) Deduce and justify (1-2 marks)
- Sample X has the higher G-C content. Each G-C pair has three hydrogen bonds whereas each A-T pair has only two, so a higher proportion of G-C pairs raises the total bonding and the separation temperature. (Sample Y, separating at the lower temperature, is comparatively A-T rich.)
- (c) Link to the Watson-Crick model (1-2 marks)
- The Watson-Crick model specifies that the strands are held by complementary base pairing with defined hydrogen-bond numbers (A-T = 2, G-C = 3). Therefore base composition directly sets the bonding energy, and separation temperature becomes a measurable consequence of structure - structure predicting a physical property is exactly what makes the model so powerful.
Top-band answers state the bond numbers explicitly, use the term "hydrogen bonds" (not vague "bonds"), and connect (c) back to the model rather than restating (a).
exam7 marksEvaluate the claim that the discovery of the structure of DNA was the work of Watson and Crick alone. In your response, refer to specific contributions and the nature of scientific evidence.Show worked solution →
"Evaluate" requires a judgement supported by evidence weighed on both sides. A Band 6 response reaches a clear conclusion, not just a list of contributors.
- Case that Watson and Crick were central (1-2 marks)
- Watson and Crick built the first correct physical model of the double helix (1953) and published it in Nature; the conceptual leap of antiparallel strands with complementary base pairing was theirs.
- Case against "alone" - the evidence base (2-3 marks)
- Their model depended on data they did not generate: Franklin's Photograph 51 supplied the helical dimensions and base spacing, and Chargaff's rules (, ) constrained the base pairing. Wilkins provided access to the King's College X-ray work. Without this experimental evidence the model could not have been built or validated.
- Nature of scientific evidence (1 mark)
- Model-building in science is an interpretation of experimental data; a model is only accepted because it explains existing evidence (here, diffraction patterns and base ratios). This makes the data-generators integral to the discovery, not peripheral.
- Judgement (1-2 marks)
- A supported conclusion: the claim is not justified - the discovery was a collaborative achievement in which Watson and Crick synthesised and interpreted the essential experimental contributions of Franklin, Wilkins and Chargaff. An answer that lists contributions but gives no explicit evaluative judgement caps at 5 marks.