Discovering the Structure of DNA
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Read This!!
DNA was first discovered in 1869, but not much was known about the molecule until the 1920s. Early
researchers discovered that DNA was comprised of repeated units called nucleotides. Each
nucleotide contains a part called a nitrogenous base. There are four different nitrogenous bases
found in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T).!
In the 1920s it was believed that these nitrogenous bases occurred in all living things in the same
repeated pattern, such as ATGC-ATGC-ATGC. If this were true, then DNA could not be the genetic
material for life. With the same repeated pattern in all species, DNA could not provide the variety
needed for a molecule containing the genetic code.!
After World War II, the biochemist Erwin Chargaff made some major discoveries about the
nitrogenous bases in DNA. His research revealed the percentage of each base (A, T, G, and C) found
in an organism’s DNA. Table 1 below includes some of Chargaff’s data and some more recent
additions.!
Model 1 — Data Analysis of Nitrogenous Bases!
Table 1. Nitrogen Base Make-Up of Different Organisms’ DNA (in Percentages)
Organism
A
T
G
C
Mycobacterium tuberculosis
15.1
14.6
34.9
35.4
Yeast
31.3
32.9
18.7
17.1
Wheat
27.3
27.1
22.7
22.8
Sea Urchin
32.8
32.1
17.7
17.3
Marine crab
47.3
47.3
2.7
2.7
Turtle
29.7
27.9
22.0
21.3
Rat
28.6
28.4
21.4
21.5
Human
30.9
29.4
19.9
19.8
1. Refer to Table 1 in Model 1.!
a. What observations can you make about the data in Table 1? What patterns do you notice?!
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b. What mathematical calculations could you make with the data that would reveal more
information about the important patterns? Make calculations for at least four different organisms
and record your results in a table below.!
c. What does the data in Table 1 show about the make-up of DNA for different species? Does this
support the idea that DNA could be the genetic material? Explain.!
DisoveringStructureDNA
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Discovering the Structure of DNA
Model 2 — Molecular Structure of Nitrogenous Bases of DNA!
Read This!!
DNA Nucleotides are made up of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
The four different nucleotides differ only in their nitrogenous bases. Figure 1 in Model 2 shows the
structure of DNA’s four nitrogenous bases. The bases are of two types. Thymine and cytosine are
single-ring structures called pyrimidines. Adenine and guanine are larger, double ring structures
called purines. !
2. For at least four different organisms in Table 4.2, calculate the ratio of purines to pyrimidines and
organize your results in a table below.!
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3. What can you conclude about the
purine:pyrimidine make-up of DNA?!
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DisoveringStructureDNA
Model 2 — Data Analysis of Nitrogen Bases
Figure 1
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Discovering the Structure of DNA
Model 3— Piecing it Together, The Double Helix!
4. Chargaff’s data was a central piece of evidence used by James Watson and Francis Crick in 1953
to successfully describe the structure of DNA. Refer to Figure 2 in Model 3. !
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a. What do you notice about the arrangement of the nitrogen bases?
Record as many observations as you can.!
Model 3 — Piecing
it Together, The
Double Helix
b. How do you think Chargaff’s data helped Watson and Crick to predict
what DNA looks like this?!
Model 4 — Photo 51 !
Read This!!
Photo 51 is the nick name given to a famous X-ray diffraction image of DNA taken by
Rosalind Franklin in May of 1952. When James Watson saw this image he
experienced a eureka moment. He later stated, “The instant I saw the picture my jaw
fell open and my pulse began to race.” Photo 51 provided Watson and Francis Crick
with crucial data that allowed them to determine the structure of DNA.!
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5. Use the kit provided to build a molecule of DNA at least 10 nucleotides long.
Then use this model of a molecule of DNA to demonstrate what is seen in
Photo 51 (Figure 3).!
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! Model 4 — Photo 51
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Figure 2
© Franklin, R. and Gosling, R.G./Nature
Figure 3
DisoveringStructureDNA
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