Chargaff’s Base-Pairing Rules
- Advanced
Douglas Wilkin, Ph.D.
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Printed: March 18, 2015
AUTHOR
Douglas Wilkin, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Chargaff’s Base-Pairing Rules - Advanced
1
Chargaff’s Base-Pairing
Rules - Advanced
• Discuss the findings of Chargaff.
• Describe the importance of the finding that in DNA, the amount of adenine and thymine were about the same
and that the amount of guanine and cytosine were about the same.
How do these four structures form DNA?
In an extremely elegant model, that’s how. The model of DNA predicts how the DNA sequence can code for proteins,
and how the molecule can be replicated. But one initial significant step was to understand the base-pairing rules.
Chargaff’s Rules
It was known that DNA is composed of nucleotides, each of which contains a nitrogen-containing base, a fivecarbon sugar (deoxyribose), and a phosphate group. In these nucleotides, there is one of the four possible bases:
adenine (A), guanine (G), cytosine (C), or thymine (T) ( Figure 1.1). Adenine and guanine are purine bases, and
cytosine and thymine are pyrimidine bases.
Erwin Chargaff (1905-2002), an Austrian-American biochemist from Columbia University, analyzed the base composition of the DNA of various species. This led him to propose two main rules that have been appropriately named
Chargaff’s rules.
Rule 1
Chargaff determined that in DNA, the amount of one base, a purine, always approximately equals the amount of a
particular second base, a pyrimidine. Specifically, that in any double-stranded DNA the number of guanine units
1
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FIGURE 1.1
Chemical structure of the four nitrogenous
bases in DNA. Notice how the purines (A
and G) are composed of two ring structures, whereas the pyrimidines (T and C)
are composed of one ring structure. The
DNA of all species has the same four
nitrogen bases.
equals approximately the the number of cytosine units and the number of adenine units equals approximately the
number of thymine units.
Human DNA is 30.9% A and 29.4% T, 19.9% G and 19.8% C. The rule constitutes the basis of base pairs in the
DNA double helix: A always pairs with T, and G always pairs with C. He also demonstrated that the number of
purines (A+G) always approximates the number of pyrimidines (T+C), an obvious consequence of the base-pairing
nature of the DNA double helix.
Rule 2
In 1947 Chargaff showed that the composition of DNA, in terms of the relative amounts of the A, C, G and T bases,
varied from one species to another. This molecular diversity added evidence that DNA could be the genetic material.
Samples
The following Table 1.1 is a representation of Erwin Chargaff’s 1952 data. This table lists the base composition of
DNA from various organisms. Data from this table supports both of Chargaff’s rules.
TABLE 1.1: Chargaff’s 1952 Data
Organism %A
E. coli
24.7
Yeast
31.3
Maize
26.8
Wheat
27.3
Grasshopper29.3
Sea
32.8
Urchin
Octopus
33.2
Chicken
28.0
Rat
28.6
Human
29.3
%G
26.0
18.7
22.8
22.7
20.5
17.7
%C
25.7
17.1
23.2
22.8
20.7
17.3
%T
23.6
32.9
27.2
27.1
29.3
32.1
A/T
1.05
0.95
0.99
1.01
1.00
1.02
G/C
1.01
1.09
0.98
1.00
0.99
1.02
%GC
51.7
35.8
46.1
45.5
41.2
35.0
%AT
48.3
64.4
54.0
54.4
58.6
64.9
17.6
22.0
21.4
20.7
17.6
21.6
20.5
20.0
31.6
28.4
28.4
30.0
1.05
0.99
1.01
0.98
1.00
1.02
1.00
1.04
35.2
43.7
42.9
40.7
64.8
56.4
57.0
59.3
Vocabulary
• adenine: A purine base found in DNA paired with thymine and in RNA paired with uracil.
2
• base pair: Two nitrogenous bases on opposite complementary DNA strands; connected by hydrogen bonds;
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Chapter 1. Chargaff’s Base-Pairing Rules - Advanced
• purine: Nitrogenous bases consisting of two ring structures; adenine and guanine.
• pyrimidine: Nitrogenous bases consisting of one ring structure; thymine and cytosine.
• thymine: A pyrimidine base found in DNA paired with adenine.
Summary
• Chargaff’s rule 1 is that the number of guanine units approximately equals the number of cytosine units and
the number of adenine units approximately equals the number of thymine units.
• Chargaff’s rule 2 is that the composition of DNA varied from one species to another.
• The base paring rules state that A always pairs with T and G always pairs with C.
Explore More
Use this resource to answer the questions that follow.
• Chargaff’s Rules: the Work of Erwin Chargaff at http://www.jbc.org/content/280/24/e21
1. What inspired Erwin Chargaff to study the chemistry of nucleic acids?
2. State Chargaff’s two rules.
3. How do Chargaff’s rules influence the structure of DNA?
Review
1. Explain the contribution of Erwin Chargaff to the identification of DNA as the heredity material.
2. What are the base pairing rules?
3. What is a purine and a pyrimidine? Name them.
References
1. Hana Zavadska. CK-12 Foundation . CC BY-NC 3.0
3