Chargaff's Rules - Online Tutor, Practice Problems & Exam Prep

Erwin Chargaff's discoveries in the 1950s revealed that DNA base composition varies among species, with adenine (A) pairing with thymine (T) and guanine (G) pairing with cytosine (C). This led to Chargaff's rules, stating that the percentages of A and T, as well as G and C, are roughly equal across different organisms. These findings were crucial for understanding the double helix structure of DNA, highlighting the significance of complementary base pairing in genetic material.

Chargaff's Rules

In this video, we're going to introduce Chargaff's rules. Way back in the early 1950s, a scientist named Erwin Chargaff made two very important discoveries related to DNA. We've got these two very important discoveries numbered down below, number 1 and number 2.

The first very important discovery that Chargaff made was that DNA base composition actually varies among different species. Different species of organisms will have different DNA compositions and different percentages of bases, adenines, thymines, cytosines, and guanines: A's, T's, C's, and G's. The second very important discovery that Chargaff made was that for each of these different species that he looked at, he noticed that the percentage of A's and T's or adenine and thymine bases were roughly equal to each other, as were the percentages of C's and G's. Today, we know that the percentage of A's and T's are roughly equal because A's are complementary base pairing with T's on opposite strands, and the percentage of G's and C's are going to be roughly equal because G's will base pair with C's on opposite strands.

However, back in the early 1950s, the structure of DNA was unknown; they did not know the structure of DNA. This important information was critical because it helped to reveal the structure of DNA later. If we take a look at our example below, we're showing you a snippet of some of Chargaff's data. What you'll notice is that we've got a table here of the DNA composition in different species. We've got the species on the left-hand side and then the percent composition of the DNA in terms of the percentages of adenines, thymine, guanine, and cytosine or A's, T's, G's, and C's in each of the species.

What you'll notice is that Homo sapiens, humans, they have a very specific percent composition of DNA, which varies from Escherichia coli, which has a different percentage of bases, a different composition of bases. The very first discovery that Chargaff made was just that, different species are going to vary in their DNA base composition. The second thing he noticed again is that it doesn't matter which species you look at, the percentage of A's and T's are roughly equal and the percentage of G's and C's are roughly equal. Looking at humans, the percentage of A is approximately equal to the percentage of T; they're roughly in the 31% ballpark. The percentage of G's is roughly equal to the percentage of C's; they are roughly in the 19-18% percent ballpark range. Again, it doesn't matter which species you look at. The percentage of A's and T's are going to be roughly equal, and the percentage of G's and C's are going to be roughly equal.

The reason that these percentages are not exactly the same is because of limitations in Chargaff's techniques that he was using. The reason why A's is not exactly equal to T's again has to do with just some limitations in these techniques. But this was very, very important information to help reveal the structure of DNA. This concludes our introduction to Chargaff's rules, these two discoveries here, and we'll be able to get some practice applying these concepts as we move forward in our course. So, I'll see you all in our next video.

Cytosine (C) makes up 42% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine (T)?

• A. 8%
• B. 16%
• C. 21%
• D. 60%

Thymine (T) makes up 28% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be guanine (G)?

• A. 14%
• B. 56%
• C. 22%
• D. 72%

Here’s what students ask on this topic:

What are Chargaff's rules and why are they important?

Chargaff's rules, discovered by Erwin Chargaff in the 1950s, state that in any given DNA molecule, the amount of adenine (A) is roughly equal to thymine (T), and the amount of guanine (G) is roughly equal to cytosine (C). These rules are important because they provided critical insights into the structure of DNA, specifically the concept of complementary base pairing. This understanding was pivotal in revealing the double helix structure of DNA, which is fundamental to modern genetics and molecular biology. Chargaff's findings laid the groundwork for understanding how genetic information is stored and replicated.

How did Chargaff's rules contribute to the discovery of the DNA double helix structure?

Chargaff's rules contributed to the discovery of the DNA double helix structure by demonstrating that the amounts of adenine (A) and thymine (T) are roughly equal, as are the amounts of guanine (G) and cytosine (C). This suggested that A pairs with T and G pairs with C, leading to the concept of complementary base pairing. This information was crucial for James Watson and Francis Crick, who used it to model the double helix structure of DNA. The complementary base pairing explained how DNA could replicate accurately, as each strand serves as a template for the formation of a new complementary strand.

What did Erwin Chargaff discover about DNA base composition among different species?

Erwin Chargaff discovered that DNA base composition varies among different species. He found that the percentages of the four nitrogenous bases—adenine (A), thymine (T), guanine (G), and cytosine (C)—differ from one species to another. However, within any given species, the amount of adenine is roughly equal to thymine, and the amount of guanine is roughly equal to cytosine. This variability among species and consistency within species were key observations that helped to understand the genetic diversity and the fundamental principles of DNA structure.

Why are the percentages of adenine and thymine, and guanine and cytosine, roughly equal in DNA?

The percentages of adenine (A) and thymine (T), and guanine (G) and cytosine (C), are roughly equal in DNA because of the principle of complementary base pairing. In the DNA double helix, A pairs with T through two hydrogen bonds, and G pairs with C through three hydrogen bonds. This pairing ensures that for every A in one strand, there is a T in the complementary strand, and for every G, there is a C. This complementary base pairing is essential for the accurate replication and transcription of genetic information.

How did Chargaff's findings challenge the tetranucleotide hypothesis?

Chargaff's findings challenged the tetranucleotide hypothesis, which proposed that DNA was composed of repeating units of four nucleotides in equal amounts. Chargaff demonstrated that the base composition of DNA varies among species and that the amounts of adenine (A) and thymine (T) are roughly equal, as are the amounts of guanine (G) and cytosine (C). This disproved the idea of a simple repeating structure and suggested a more complex and variable composition, paving the way for the discovery of the double helix structure and the understanding of DNA as a carrier of genetic information.