Chargaff’s Data Inquiry Background Information:
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 call nucleotides. Each nucleotide contains a part called a nitrogen base. There are four nitrogen bases in DNA: Adenine (A), Cytosine (C), Guanine (G), & Thymine (T).
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. If this were true, then DNA could not be the hereditary molecule. With the same repeated pattern in all species, DNA could not provide the variety needed for a molecule containing the genetic code for each living thing.
In the 11940’s, the biochemist Erwin Chargaff made some major discoveries about the nitrogen bases in DNA. His research revealed the percentage of each base (A, T, C, and G) found in an organism’s DNA. This NEW data changed how they viewed DNA. The table below includes some of Chargaff’s data and some more recent additions.
Data:
Nitrogen Base Make-Up of Different Organisms’ DNA (in percentages)
Organism
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (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

Use the Background Information and Data to answer the following questions
Make at least 3 observations about the data in the table. What patterns do you notice?

What do you think the data shows about the make-up of DNA for different species? Provide data evidence for why you think that.

After seeing the new data in the 1940s, what do you think researchers concluded about DNA’s potential to carry the genetic code? Provide at least one evidence to support that conclusion.

Adenine and guanine are similarly shaped nitrogen bases called purines. Thymine and cytosine are similar in shape and classified as pyrimidines. What can you conclude about the purine:pyrimidine makeup of DNA? Provide evidence.

Piecing it Together
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.
Look at the diagram of the DNA molecule that has labeled nitrogen bases.

Use the DNA diagram to answer the following questions
Make at least 3 observations about the DNA molecule diagram.

Does Chargaff’s data support the current diagram of DNA? Provide evidence to support your answer.

Nitrogen bases that match up are called complementary base pairs. List out the two complementary base pairs.

Predict what the matching side of complementary bases would be if one side of DNA is A T G G C T A A