Experiments carried out in the first half of the twentieth century led scientists to the view that DNA was the genetic material that contained the instructions for life (not proteins as had been previously thought). Erwin Chargaff (1905–2002), working in the USA, analysed the components of DNA taken from different organisms – animals, plants and bacteria. He measured the proportions of adenine, guanine, cytosine and thymine. Some of his results are shown below.

%A / %G / %C / %T
maize / 26.8 / 22.8 / 23.2 / 27.2
octopus / 33.2 / 17.6 / 17.6 / 31.6
chicken / 28.0 / 22.0 / 21.6 / 28.4
wheat / 27.3 / 22.7 / 22.8 / 27.1
human / 29.3 / 20.7 / 20.0 / 30.0

1aWork out the ratios A/T and C/G for the organisms shown in the table.

bWhat conclusion can you draw?

2‘Chargaff’s results provided evidence for the base-pair model of DNA.’
Explain whether you agree or disagree with this statement.

3aWork out the ratio of the percentage of Gs and Cs compared to the percentage of As and Ts for:

imaize

iioctopus.

bWhat conclusion do you think Chargaff drew from these ratios?

By the 1950s, a significant amount of research had been done on DNA, but the structure was not determined until Watson and Crick developed the double helix model. In doing this they used a lot of secondary data.

The Crick and Watson model of DNA had these features:

  • two sugar-phosphate strands
  • the two strands twisted together in a double helix
  • base pairs forming ‘cross pieces’ between the strands
  • adenine always linked to thymine; and cytosine to guanine

1For each of the features above, identify which pieces of evidence or hypotheses they used from the list below.

ADNA contains four bases.

BThe bases are connected at right angles to the strands.

CEqual proportions of A and T; equal proportions of C and G (Chargaff’s rule)

DThe strands are made of ribose (a sugar) and phosphate.

EPauling’s research of helical parts of proteins

FFurberg’s DNA model (putting bases on the inside)

GFranklin’s X-ray images

HFranklin’s assertion that bases on the outside wouldn’t be stable

2Crick’s and Watson’s role in discovering the structure of DNA is very high profile, but in fact they used a lot of secondary data (as we have seen above). Comment on the benefits and pitfalls of using secondary data as opposed to primary data.

Look at the diagram of the structure of DNA and read the text below. Then work (in pairs) to decide which statements are true and which are false.

In the 1950s, scientists were trying to find out about DNA ‘the building block of life’. They knew that DNA was responsible for passing on genetic information, but they did not know how it was done. They wanted to work out its structure so that they could understand this process.

The scientists Maurice Wilkins and Rosalind Franklin took X-ray photographs of DNA, which showed that it had a regular structure. Wilkins thought that the images showed that the structure was a helix (spiral).

Crick and Watson suggested that the structure had a sugar–phosphate central helix (like a backbone) with the bases A, C, T and G sticking out of it. Franklin told them that this wouldn’t work – the molecule would fall apart.

Then in 1953, Wilkins showed Crick and Watson an image that Franklin had taken (called photo 51). This photograph led them to the idea that the structure was a double helix, with the bases in the centre like rungs on a ladder.

In 1962, Wilkins, Crick and Watson were awarded the Nobel Prize, but sadly Franklin had died 4 years earlier at the age of 37.

Now decide which of these statements are true and which false:

aDNA is responsible for passing on genetic information.T / F

bThe DNA molecule has one central helix.T / F

cDNA is made up of sugar, phosphate and four bases (A, C, T and G)T / F

dWilkins and Franklin’s X-ray photos of DNA helped in determining the structureT / F

eCrick and Watson discovered the structure of DNA with no help from other scientistsT / F

fCrick and Watson were the first scientists to suggest that DNA had a helix structureT / F

gPhoto 51 was important in determining the double helix structureT / F

hFranklin agreed with other scientists that the DNA molecule had a central sugar–phosphate ‘backbone’ T / F

iCrick, Watson and Wilkins were awarded the Nobel Prize for their work on DNA.T / F