Question

The DNA from sea urchins contains about \(32 \% \mathrm{~A}\) and about \(18 \%\) G. What percentages of \(T\) and \(C\) would you expect in sea urchin DNA? Explain.

Short answer

%T = 32% and %C = 18%.

Step-by-step solution

Understand the Composition of DNA

DNA consists of four nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C). According to Chargaff's rules, the amount of adenine in DNA is equal to the amount of thymine, and the amount of guanine is equal to the amount of cytosine. Thus, %A = %T and %G = %C.

Define Known Quantities

From the problem statement, we know that the percentage of adenine (A) in the DNA is 32% and the percentage of guanine (G) is 18%. Using Chargaff's rules, we can deduce that %T = %A and %C = %G.

Calculate %T

Since Chargaff's rule tells us that %A = %T, and we know %A = 32%, it follows that the percentage of thymine (T) is also 32%.

Calculate %C

Similarly, since %G = %C as per Chargaff's rules, and we know %G = 18%, the percentage of cytosine (C) is also 18%.

Verify Total Composition

To ensure consistency, check that the total percentage of all nucleotides sums up to 100%: %A + %T + %G + %C = 32% + 32% + 18% + 18% = 100%. This confirms the calculations are correct.

Key concepts

Nucleotides

Nucleotides are the basic building blocks of DNA, responsible for storing the vast genetic information in living organisms. Each nucleotide consists of three components: a sugar molecule, a phosphate group, and a nitrogenous base. The sugar in DNA nucleotides is deoxyribose, and it is connected to both the phosphate group and one of four types of nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C).
These bases are critical for the encoding of genetic information as they form the "rungs" of the DNA double helix ladder through specific pairing, following particular rules. The sequence of these nucleotides constitutes genes, leading to the production of proteins via the processes of transcription and translation.

• Deoxyribose: The sugar component in DNA nucleotides
• Phosphate group: Links sugars to form the DNA backbone
• Nitrogenous bases: A, T, G, C – responsible for storing information

Adenine and Thymine

Within the DNA structure, adenine (A) and thymine (T) form a pair through two hydrogen bonds. This specific pairing is essential for DNA's functionality and stability. The rule that adenine always pairs with thymine is part of the base pairing rules crucial for maintaining the accuracy of DNA replication and transcription.
Adenine and thymine pairing allows for the consistency needed when DNA is copied during cell division.

• Pairing: A pairs with T through two hydrogen bonds
• Role in DNA: Ensures stability and fidelity during replication

When we say that %A = %T, it means that in a given DNA molecule, the proportion of adenine will always match that of thymine, maintaining a balanced structure. If sea urchin DNA contains 32% adenine, it similarly contains 32% thymine.

Guanine and Cytosine

Just like adenine and thymine, guanine (G) and cytosine (C) also form pairs, but they do so through three hydrogen bonds. This stronger bonding makes G-C pairs more robust than A-T pairs. This pairing rule, where %G = %C, is equally crucial in defining the DNA structure's stability and function.
The enhanced bonding in G-C pairs is crucial in regions of the DNA which require higher stability, such as those impacting gene regulation and structural integrity.

• Pairing: G pairs with C through three hydrogen bonds
• Role in DNA: Provides higher stability due to more bonds

For sea urchin DNA, since there is 18% guanine, there will likewise be 18% cytosine, aligning with Chargaff's rules.

Chargaff's Rules

Chargaff's rules are fundamental principles that describe how nucleotides pair up in DNA. Proposed by Erwin Chargaff, these rules state that in the DNA of any given species, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C).
These rules reflect the base-pairing mechanism of DNA and are crucial to understanding its double-helix structure.

• Base Pair Equality: %A = %T and %G = %C
• Universality: Chargaff's rules apply to nearly all DNA
• Structural implication: Ensures the integrity and uniformity of the DNA molecule

In the case of sea urchin DNA, Chargaff's rules help us deduce that the percentage of thymine is equal to adenine at 32%, and cytosine is equal to guanine at 18%, allowing the total nucleotide composition to make a complete 100%.