Problem 78 - Chemistry: The Central Science

Question

When samples of double-stranded DNA are analyzed, the quantity of adenine present equals that of thymine. Similarly, the quantity of guanine equals that of cytosine. Explain the significance of these observations.

Step-by-Step Solution

Answer

The equal quantities of adenine (A) and thymine (T), and guanine (G) and cytosine (C) in double-stranded DNA are a result of the complementary base pairing rule. This rule plays a crucial role in ensuring the stability of the DNA double helix structure, enabling accurate DNA replication during cell division, and maintaining uniform structural stability across the entire DNA molecule. The complementary base pairing of A with T and G with C is essential for proper DNA function and information storage.

Step 1: Understand the structure of DNA

Double-stranded DNA consists of two long chains of nucleotides that are twisted around each other to form a double helix structure. Each nucleotide is composed of a sugar molecule, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C). The two strands of DNA are held together by hydrogen bonds formed between the nitrogenous bases.

Step 2: Explain complementary base pairing

The key feature of the DNA structure is the complementary base pairing, which means that adenine (A) always pairs with thymine (T) and guanine (G) always pairs with cytosine (C). The reason for this complementary base pairing is that the specific hydrogen bond formation between these pairs allows for the most stable double helix structure. In each A-T pair, there are two hydrogen bonds, while in each G-C pair, there are three hydrogen bonds.

Step 3: Discuss the significance of equal quantities of A-T and G-C pairs

The observation that the quantity of adenine (A) present equals that of thymine (T) and the quantity of guanine (G) present equals that of cytosine (C) in samples of double-stranded DNA is a direct consequence of the complementary base pairing rule. Since each adenine (A) must pair with a thymine (T), their quantities must be equal in a double-stranded DNA molecule. Similarly, since each guanine (G) must pair with a cytosine (C), their quantities must also be equal. This complementary base pairing has several important implications for the properties, stability, and function of DNA: 1. It ensures that the two strands of DNA are held together in a stable double helix structure, allowing DNA to store genetic information securely. 2. Complementary base pairing allows for accurate replication of DNA during cell division. When replication occurs, the two strands of the double helix separate, and each strand serves as a template for the synthesis of a new complementary strand. Because each base can only pair with its specific complementary base, the replication is highly accurate. 3. The equal quantities of A-T and G-C pairs also have implications for the uniformity of the DNA double helix. Since A-T pairs have two hydrogen bonds and G-C pairs have three hydrogen bonds, an unequal distribution of these pairs could result in regions with different structural stability throughout the DNA molecule. Equal quantities of A-T and G-C pairs ensure that the stability and helical structure remain uniform across the entire DNA molecule. In conclusion, the observation of equal quantities of A-T and G-C pairs in double-stranded DNA is a direct consequence and a clear indication of the complementary base pairing rule, which ensures the stability and function of the DNA molecule.