1 DNA discovery

DISCOVERING THE STRUCTURE OF 

X-ray 

photograph of 

DNA taken by 

Rosalind Franklin 

at King’s 

College London 

DNA 

THE DISCOVERY OF THE STRUCTURE OF DNA 

as announced by Francis Crick and James Watson in their 1953 paper A Structure for Deoxyribose Nucleic 

Acid published in Nature was a landmark moment in biology...but who else contributed to this discovery? 

SPECIFICATION CONTENT 

Old: Know the different approaches used 

by Crick and Watson, (limited to modelling) and 

Franklin, (limited to X-ray crystallography) when 

researching the structure of DNA. New: 

Describe (in outline only) how the work of 

Chargaff, Franklin and Wilkins, and Watson and 

Crick, using different lines of evidence, led to 

the discovery of DNA; know that the 

development of the scientific theory of the 

structure of DNA is an example of the 

collaborative nature of science, and how new 

scientific knowledge is validated (for example 

peer review). 

STARTER 

Play kinaesthetic true or false. Read 

statements and ask class to make a T (stand 

up, arms out) if the answer is true, or an X 

(stand up, arms crossed) if the answer is false. 

Alternatively, to focus on how science works, 

use the ‘would you rather...?’ activity. 

STIMULUS 

Begin by asking students to read about 6 

contributions to the discovery of the structure 

of DNA. Working in 3s, ask them to rank the 

contributions in order of importance on the 

pyramid grid. Discuss justifications. Have a 

colleague deliver the letter to the problem page. 

Ask students to discuss letter in pairs. 

QUESTIONS 

Allow some individual thinking time, then 

ask students to write the question they think it 

is important to address on a post it. Share in 

pairs, then snowball to 4s. Each 4 should write 

one question on A4 paper, and place in the 

centre of circle. Read all questions aloud then 

ask students to vote for the most important 

question by placing their post it on that 

question. Rank the questions from most to 

least important, as an agenda for the lesson. 

ENQUIRY 

Questions that the teacher can use to 

provoke thought are provided on the COSE 

guidance page. 

EVALUATION 

Find out what students thought about the 

enquiry by laying the emoticons on the floor 

and asking them to choose the face that best 

Crick’s sketch of DNA 

represents their feelings about the enquiry. 

Ask students what they need to work on 

for next time. Write this on the target board to 

use as a reminder next enquiry. 

NEXT STEPS 

Task students to reply to the letter, 

answering the question What does the story of 

the discovery of DNA tell us about how science 

works? using examples to back up their ideas. 

This should be handed in in the envelope 

provided. 

www.ulster.ac.uk/scienceinsociety/ [ 1 ] Resource for Teachers 

Images courtesy of the Wellcome Library

T R U E 

Each body 

cell contains only the 

genetic information 

needed to do its job. 

F 

Only sex cells 

contain 

chromosomes. 

F 

Genes 

determine 

everything about 

you. 

DNA is a long 

molecule. 

Bacteria and 

fungi don’t have 

DNA. 

F F 

DNA is made 

from 

chromosomes. 

F 

DNA is made 

from genes. 

F 

Genes are 

made from DNA. 

T T 

F A L s E 

www.ulster.ac.uk/scienceinsociety/ [ 2 ] Resource for Teachers

WOULD YOU RATHER...? 

Print the pairs of statements below and, taking one at a time, ask students to stand next to 

the one they would rather. Share and evaluate the quality of reasons. In each case, ask 

whether the choices are mutually exclusive and ask how they link to the DNA discovery story. 

compete v collaborate 

win at all costs 

be recognised 

for your work 

be accurate and 

correct but slow 

deal face-to-face with 

problems with people 

ask for something 

and risk refusal 

v 

lose but keep your 

principles intact 

v 

v 

v 

v 

remain 

anonymous 

be fast but miss 

details 

ignore problems 

with people 

take something 

without consent 


1 

3 

2 

4 5 6 


the discovery pyramid 

Instructions 

Read the 6 cards that describe the contributions of different 

people to the discovery of DNA. Rank the contributions, with the 

most important placed at the top of the grid and the least along 

the base. What are your justifications for this order? 

Friedrich Miescher was the first person to 

isolate the chemical we now know to be 

DNA from cells. He had been trying to 

purify proteins in white blood cells 

(collected from pus in old bandages sent 

to him from a clinic) but noticed a 

chemical that did not behave like a 

protein. He realised he had discovered a 

new chemical (DNA) and later found the 

same chemical in other types of cell. 

Linus Pauling used a new method of 

model-making to create 3D molecular 

structures to propose (incorrectly) that 

DNA was a triple helix. He built his model 

using known molecular distances and 

bond angles, with the knowledge that 

helical molecular structures were possible. 

Maurice Wilkins initial work on DNA 

showed that it could be crystallised for 

study by X-ray diffraction. He discussed 

the structure of DNA with Crick and 

Watson, showing them Franklinʼs image 

and helping them to interpret it. He later 

used X-ray crystallography to confirm and 

refine Watson and Crickʼs double helical 

structure for DNA. 

Erwin Chargaff discovered, using paper 

chromatography, that DNA had a different 

composition in different species: the bases 

appeared in a different order. He also found 

that no matter what species DNA came from, 

the number of purines was equal to the 

number of pyrimidines, and in particular that 

the amount of adenine (A) was equal to the 

amount of thymine (T) and the amount of 

guanine (G) was always equal to the amount 

of cytosine (C). 

Rosalind Franklin studied DNA using Xray 

crystallography. She prepared DNA 

samples and took the X-ray photograph 

that demonstrated that DNA was a helix in 

shape. She deduced the dimensions of 

DNA strands and that the phosphate 

groups were on the outside of the 

molecule. Her X-ray photo was shown to 

Crick and Watson, who said that it was 

key to them discovering the structure of 

DNA when they did. 

James Watson and Francis Crick used 

Pauling’s modelling method and Franklin’s Xray 

photograph and measurements of DNA 

(given to them by Wilkins without her consent) 

to solve the structure of DNA. Using card cutouts, 

they built DNA as a helix containing two 

strands connected with hydrogen bonds. 

Bases (A, T, G, and C) are attached to sugars 

on a backbone. The backbone is made of 

sugars and phosphate groups. The ‘rungs’ of 

DNA are made of bases. A is always paired 

with T and G is always paired with C. 


Dear Auntie Anna, 

I fear my scientific career is over before it has 

even begun. 

I have been finding out about the discovery of DNA 

and it seems that in the race to find things out 

scientists will do anything to get the data they need, 

even cheat. Look at Rosalind Franklin - she didn’t 

get the Nobel Prize with Crick, Watson and Wilkins 

even though her data was crucial to the discovery 

of DNA’s structure. 

Can’t scientists work together for the common 

good? 

I am thinking of giving up science for something 

more collaborative. 

Please help! 

RJ 


evaluating with emoticons 

Cut out and laminate the emoticons below and place them face-up on the floor (you may need multiple copies). Ask students to 

reflect on how they felt during the enquiry, and why, and ask them to select the emoticon that best represents their feelings. 

Discuss as a class a) what was positive about the enquiry and b) how the group could improve their enquiry skills. You may 

want to provide blank laminated paper and a marker for students who cannot find an appropriate emoticon. 


TARGET: 

The more positive you feel about how we hit this target 

the closer to the centre you place your sticker. 


the 

Miescher 

In 1869, no-one knew that DNA 

existed. Friedrich Miescher wanted to find 

out which proteins were present in white 

blood cells. He asked clinics to send him 

their patients’ old bandages filled with pus, 

and he started to analyse them. However, 

he found something that wasn’t protein: it 

had much more phosphorus and wouldn’t 

break down in the same way as proteins. 

What he had discovered was DNA. 

Chargaff 

information 

Chargaff wanted to find out if there was 

any difference between the DNA of different 

species. He made 2 major discoveries: that 

the order of nucleotides (a nucleotide is a 

big molecule made from a phosphate group, 

a sugar and a base) is different in different 

species, and that DNA has properties that 

remain constant in different species: the 

amount of adenine (A) is always equal to the 

amount of thymine (T) and the amount of 

guanine (G) is always equal to the amount of 

cytosine (C). 

Franklin and Wilkins 

Francis Wilkins had the idea of studying 

DNA using X-ray crystallography, and had 

started this work at King’s College with his 

PhD student, Ray Gosling, when Rosalind 

Franklin was appointed by John Randall. 

Franklin took the clearest picture of DNA 

using X-ray diffraction (Photograph 51), and 

it was this that demonstrated that DNA had 

a helical structure. 

Crick and Watson 

Francis Crick and James Watson were 

determined to discover the structure of 

DNA. They used a molecular modelling 

technique devised by Linus Pauling to 

create a large scale model DNA in their 

laboratory. They used the structure of the 

constituent parts and known atomic 

distances to build their model. 

Serendipity 

Crick and Watson shared an office with 

Jerry Donohue, a theoretical chemist. Crick 

and Watson were using published 

information about the structure of the bases, 

but Donohue was able to tell them that this 

was likely to be incorrect. Using the revised 

structural information, along with the X-ray 

data that had obtained from INSERT, Crick 

and Watson were able to revise their 

structure so that it obeyed Chargaff’s rule 

and so that complementary bases fitted 

together (A with T and G with C). They 

found that DNA is a double helix made of a 

sugar-phosphate backbone and 

complementary base pairs 

DNA discovery 

Publication 

Watson and Crick published their 

double helical structure of DNA in Nature in 

1953, along with supporting articles by 

Wilkins, Stokes and Wilson (who used X-ray 

crystallography evidence to suggest that the 

DNA double helix exists in biological 

systems) and Franklin and Gosling (who 

provide evidence from X-ray crystallography 

to support the double helical structure with 

the phosphate backbone on the outside of 

the helix). Nature is a peer-reviewed journal. 

This means that the articles they publish 

have been scrutinised by other expert 

scientists with expertise in the subject area. 

The Nobel Prize 

In 1962, the Nobel Prize in Physiology 

or Medicine was awarded to Francis Crick, 

James Watson and Maurice Wilkins for 

“their discoveries concerning the molecular 

structure of nucleic acids and its 

significance for information transfer in living 

material.” Wilkins recognised the 

contribution of Rosalind Frankin’s X-ray 

analysis of DNA in his acceptance speech. 

Rosalind Franklin died of cervical cancer in 

www.ulster.ac.uk/scienceinsociety/ [ 9 ] Resource for Teachers 

1958. 

Images © Wellcome Library

1 DNA discovery

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