Chargaff’s DNA Data
Date ______
Name_____________________
Introduction: DNA was first
discovered in 1869, but not much
was known about the molecule until
the 1920’s. Early researchers
discovered that DNA was comprised
of repeated units called nucleotides.
1. Each nucleotide contains three
parts. Label the 3 parts of one
nucleotide:
2. Each nucleotide contains a part
called a nitrogen/nitrogenous base. There are several different types of bases. List all of the different
nitrogenous bases?
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3. In the 1920s it was believed that these nitrogen bases occurred in all living things in the same repeated
pattern, such as ATGC ATGC ATGC ATCG. If this were true, then DNA could not be the hereditary molecule.
Explain why.
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After World War II the biochemist Erwin Chargaff made some major discoveries about the nitrogen bases in
DNA. The table below includes some of Chargaff’s data. Use this data to answer the following questions
and see if you can make the same discovery.
Table I: Nitrogen Base Make-Up of Different Organisms’ DNA (in Percentages)
Organism
Bacteria
Mushroom
Sea Urchin
Marine Crab
Turtle
Rat
Human
A
15.1
31.3
32.8
47.3
28.8
28.6
30.9
T
14.6
32.9
32.1
47.3
27.9
28.4
29.4
G
34.9
18.7
17.7
2.7
22.0
21.4
19.9
C
35.4
17.1
17.3
2.7
21.3
21.5
19.8
Date ______
Chargaff’s DNA Data
Answer the following questions using the table on the previous page.
Name_____________________
4. Do you notice any patterns in the data table? If so use evidence from the table to support your point.
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5. Before concluding that the pattern seen in the data is universal, which other species would you want
to test? Why? _________________________________________________
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6. For at least 3 species in the data table, calculate the ratio of purines to pyrimidines and organize your
results in a table.
Species
A + G = Total % Purine
T + C = Total % Pyrimidine
7. What can you conclude about the purine:pyrimidine relationship within DNA?
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8. A scientist is analyzing the DNA of a frog. He discovers that about 6% of the frog’s DNA contains the
base adenine. Estimate the percentage of the DNA that contains thymine, the percentage of guanine
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and finally the percentage of cytosine.
9. How do you think Chargaff’s data helped Watson and Crick to build their model of DNA?
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10. What characteristic of life could be supported by the information gained from this discovery?
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