Problem 3 - Campbell Biology

Question

In analyzing the number of different bases in a DNA sample, which result would be consistent with the base-pairing rules? \(\begin{equation}\begin{array}{l}{\text { (A) } \mathrm{A}=\mathrm{G}} \\\ {\text { (B) } \mathrm{A}+\mathrm{G}=\mathrm{C}+\mathrm{T}} \\ {\text { (C) } \mathrm{A}+\mathrm{T}=\mathrm{G}+\mathrm{C}} \\ {\text { (D) } \mathrm{A}=\mathrm{C}}\end{array} \end{equation}\)

Step-by-Step Solution

Answer

Option (B) \(\text{A} + \text{G} = \text{C} + \text{T}\)

Step 1: Understand the Base-Pairing Rules

In DNA, the base-pairing rules state that Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This means the quantity of A should be equal to the quantity of T, and the quantity of G should be equal to the quantity of C.

Step 2: Analyze Each Option Against the Base-Pairing Rules

Now check each option provided: (A) \(\text{A} = \text{G}\): This is incorrect because A pairs with T and G pairs with C, so A does not have to be equal to G. (B) \(\text{A} + \text{G} = \text{C} + \text{T}\): This is consistent with the base-pairing rules since the total number of purines (A and G) is equal to the total number of pyrimidines (C and T). (C) \(\text{A} + \text{T} = \text{G} + \text{C}\): This is incorrect because the total number of A + T does not have to equal G + C. (D) \(\text{A} = \text{C}\): This is incorrect because A pairs with T and not with C.

Step 3: Select the Consistent Result

Based on the analysis, option (B) \(\text{A} + \text{G} = \text{C} + \text{T}\) is consistent with the base-pairing rules in a DNA sample.

Key Concepts

Adenine and Thymine pairing

In the structure of DNA, each base matches up with a specific partner. Adenine (A) always pairs with Thymine (T). This is a fundamental rule of the DNA structure and it is due to the way these molecules are structured.
The bond between Adenine and Thymine is formed through two hydrogen bonds, providing a stable yet easily separable link.
This precise pairing is crucial during DNA replication, ensuring that the genetic information is accurately copied.

• Adenine (A) = Thymine (T)
• 2 hydrogen bonds between them

Understanding this specific pairing helps explain why the quantities of these bases in a DNA molecule are always equal.

Guanine and Cytosine pairing

Just like Adenine pairs with Thymine, Guanine (G) has a specific partner too, which is Cytosine (C). Guanine and Cytosine are connected by three hydrogen bonds, making their pairing slightly stronger than that of Adenine and Thymine.
This additional bond gives more stability to sections of DNA that are rich in G-C pairs.
This stability is particularly important in regions of the genome that need to remain intact under stressful conditions.

• Guanine (G) = Cytosine (C)
• 3 hydrogen bonds between them

Thus, in any DNA sample, the number of Guanine bases will always be equal to the number of Cytosine bases.

Purines and Pyrimidines in DNA

DNA bases are divided into two categories: purines and pyrimidines. Adenine (A) and Guanine (G) are purines, which are larger molecules with a two-ring structure.
Thymine (T) and Cytosine (C) are pyrimidines, which have a smaller, single-ring structure.
This complementary structure allows for the stable formation of the double helix because a purine always pairs with a pyrimidine.

• Purines: Adenine (A), Guanine (G)
• Pyrimidines: Thymine (T), Cytosine (C)

Based on this understanding, the correct base-pairing formula is confirmed: the number of purines (A+G) always equals the number of pyrimidines (T+C).