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

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:

1. A deoxyribose sugar (a 5-carbon sugar).
2. A phosphate group attached to the 5' carbon of the sugar.
3. 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 $A = T$ and $G = C$. 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.

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.

Try this

Q1. A double-stranded DNA sample contains 18 percent guanine. Calculate the percentage of adenine, thymine and cytosine in the sample. [3 marks]

• Cue. Apply Chargaff's rules: G equals C, A equals T, and the four together total 100 percent. C is 18 percent, so A plus T is 64 percent split evenly.

Q2. Justify the inclusion of Rosalind Franklin in any historical account of the DNA double helix. [3 marks]

• Cue. Identify Photograph 51 as the X-ray diffraction evidence that revealed the helical pitch and base spacing, and note that Watson and Crick relied on this data to build their 1953 model.

Q3. A bacterial DNA sample melts (the two strands separate) at 95 degrees C. A second sample from a different organism melts at 88 degrees C. (a) Account for the difference in melting temperatures. (b) Predict which sample has the higher G-C content and justify your answer. [2+2 marks]

• Cue. (a) Hydrogen bonds hold the strands together, so more bonds means more energy needed. (b) Higher melting temperature points to higher G-C content because each G-C contributes three hydrogen bonds rather than two.