Chargaff's Rules: Videos & Practice Problems

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, so different species of organisms will have different DNA compositions and different percentages of nitrogenous bases; adenines, thymines, cytosines, and guanosines, denoted as 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 he looked at, he noticed that the percentage of As and T bases, or adenine and thymine nitrogenous bases, were roughly equal to each other, as were the percentages of Cs and Gs. Today, we know that the percentage of A's and T bases are roughly equal because in the structure of DNA, A's are complementary base pairing with T's on opposite strands. The percentage of G's and C's is going to be roughly equal again 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, and this important information was critical because it helped to reveal the structure of DNA later.

If we take a look at our example down below, we're showing you a little snippet of some of Chargaff's data. What you'll notice is that we've got this 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 percentage of adenines, thymines, guanosine, 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, have a specific percentage of DNA composition, 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. In humans, the percentage of A is approximately equal to the percentage of T, so they're roughly in the 31% ballpark. The percentage of G's is roughly equal to the percentage of C's. They're roughly in the 18% 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. This is really what this second important discovery is referring to.

The reason these percentages are not exactly the same is because of limitations in Chargaff's techniques that he was using. The reason why the 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 here 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%

Answer: (A)

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%

Answer: (C)

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). This is because A pairs with T and G pairs with C in the DNA double helix. These rules are important because they provided critical evidence for the double helix structure of DNA, which was later elucidated by Watson and Crick. Understanding these base pairings is fundamental for comprehending DNA replication and genetic inheritance.

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 revealing 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, providing a clue to the complementary base pairing mechanism. This information was crucial for Watson and Crick, who used it to propose the double helix model of DNA, where two strands are held together by these specific base pairs, forming a stable structure essential for genetic replication and function.

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

Erwin Chargaff discovered that the DNA base composition varies among different species. Specifically, he found that the percentages of adenine (A), thymine (T), guanine (G), and cytosine (C) differ from one species to another. This was a significant finding because it indicated that DNA is not a uniform molecule but rather has species-specific variations. This discovery helped to establish that DNA carries genetic information unique to each organism, laying the groundwork for understanding genetic diversity and evolution.

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

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

How did Chargaff's findings challenge the previously held beliefs about DNA?

Before Chargaff's findings, it was believed that DNA was a repetitive and uniform molecule, not varying significantly among different species. Chargaff's discovery that DNA base composition varies among species and that the amounts of adenine (A) and thymine (T) are roughly equal, as are guanine (G) and cytosine (C), challenged this notion. His findings suggested that DNA has a complex and specific structure, which carries genetic information unique to each organism. This was a pivotal shift in understanding DNA's role in heredity and molecular biology.