1 DNA discovery

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.
www.ulster.ac.uk/scienceinsociety/ [ 5 ] Resource for Teachers