Name:____________________________________________
Period: _______
Discovering the Structure of DNA
Clues Set 1 (1868-1952)
Note: This inspiration for and original version of this activity originally appeared in a book entitled something like
Inductive Biology. I’ve lost the book, and would love to credit the author…
My Notes: What
was discovered?
1. In 1868, Swiss biochemist Friedrich Miescher found that when pepsin (an enzyme known to
break apart proteins) was added to chromosomes, atoms of oxygen, carbon, hydrogen, and
nitrogen were detected. This made sense, because these were the atoms known to be present in
proteins. But he also detected phosphorus atoms. Because phosphorus atoms are never in
proteins, he suspected that another type of molecule, in addition to protein, must be present in
chromosomes. He named the new molecule nuclein.
2. During the 1880s, German biologist Walter Flemming studied the behavior of chromosomes in
reproducing cells. His work showed that new individuals begin with the union of sperm and egg
cells, which always contain chromosomes and often little else. This established that
chromosomes carry the genetic material,
3. During the early 1900s, German chemist Robert Feulgen discovered that all body cells of any
particular organism contain precisely the same amount of nuclein but that the amount of protein
varies from cell to cell. He also found that egg and sperm cells contain exactly one-half the
amount of nuclein present in body cells. This was not necessarily the case for the amount of
protein.
4. Later work determined that the nonprotein part of nuclein was
deoxyribonucleic acid (DNA)—but its
structure was still uncertain. It was clear
that that the monomer of DNA was a
nucleotide. Nucleotides can be broken
down into three parts: 1) a five-carbon
sugar called deoxyribose, 2) an atom of
phosphorus surrounded by four atoms of
Nucleotide: Structural formula | graphic representation
oxygen called a phosphate group, and 3)
molecules called nitrogenous bases,
which consist of either a single ring of nitrogen and carbon atoms, or a double ring. Four different
kinds of nitrogenous bases were found: adenine, guanine, cytosine, and thymine. In other words,
DNA consists of four different kinds of nucleotides, one with each of the four nitrogenous bases.
Two representations of a nucleotide containing cytosine are shown above.
5. During the late 1940s, English scientists discovered that DNA (like sugar) can crystalize when
water is removed. This fact suggested that the atoms in DNA must be arranged in a very orderly
way, perhaps with many repetitions of a fairly simple pattern. Protein, by contrast, doesn’t
crystalize.
6. By 1952, it was known that viruses attack bacterial cells by injecting them with the virus’s
genetic information. But what was that information made of? In 1952, Americans Martha Chase
and Alfred Hershey grew viruses with radioactive phosphorus in the DNA molecules and
radioactive sulfur in the protein molecules. After these viruses attacked the bacteria and injected
their genetic information, only radioactive phosphorus was found inside the bacteria(see diagram
below, and see above to remind yourself which of the molecules we’ve been discussing contains
phosphorus)
CLASS SUMMARY: Key points from Clues Set 1 (1 – 6
above)
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Clues Set 2 (1952)
7. X-ray diffraction involves the passing X rays
through a crystal and recording the ways the X-rays
are deflected (bent). The procedure is somewhat like
trying to figure out the shape of an object by looking
at its shadow (but considerably more complex). X-ray
diffraction photographs of DNA made by Rosalind
Franklin and Maurice Wilkins of Kings College,
London, indicated that the DNA molecule is shaped
like a spiral helix.
8. Evidence indicates that the helix contains strands
of phosphate groups and sugar molecules linked by
covalent chemical bonds.
9. Watson and Crick calculated that a single chain of
nucleotides would have a density only half as great
as the known density of DNA.
My notes: What was
discovered?
An X-ray photograph of DNA, taken by
Rosalind Franklin late in 1952.
Class Summary: Key points from Clues Set 2 (7 -9
above)
Clues Set 3 (1952-1953)
10. During the 1940s, Austrian biochemist Erwin Chargaff
experimentally obtained the information shown in the table
below. The implications of these data slowly were becoming
clear to Watson and Crick in the summer of 1952.
Adenine:Thymine and Guanine: Cytosine Ratios
found in DNA Molecules
My Notes: What was
discovered?
DRAWING OF YOUR DNA MODEL
Tissue and Organism Adenine Thymine Guanine Cytosine
Thymus cells
Human
30.9
29.4
19.9
19.8
Sheep
29.3
28.3
21.4
21.0
Pig
30.9
29.4
19.9
19.8
Spleen cells
Human
29.2
29.4
21.0
20.4
Sheep
28.0
28.6
22.3
21.1
Pig
29.6
29.7
20.4
20.8
Liver cells
Human
30.3
30.3
19.5
19.9
Sheep
29.3
29.2
20.7
20.8
Pig
29.4
29.7
20.5
20.5
Non-animal cells
E. coli bacterium
26.0
23.9
24.9
25.2
Yeast
31.3
32.9
18.7
17.1
11. In the spring of 1953, Watson arrived at the lab early one day, cut
out cardboard models of adenine, thymine, guanine, and cytosine
molecules, and began arranging them in various combinations and
patterns on his desk. He discovered that an adenine-thymine pair
presumably held together by relatively weak hydrogen bonds is
identical in shape to a guanine-cytosine pair also held together by
hydrogen bonds.
12. Later that same day, Watson showed Crick his result, and when
they went to lunch at the Eagle Cafe in London, Crick told everyone
within hearing distance that they had found the secret of life.
Class summary from Clues Set 3 (10 – 12 above)
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