If a DNA double helix is 100 nucleotide pairs long and contains 25 adenine bases, how many guanine bases does it contain?
The Correct Answer and Explanation is :
To solve this problem, we need to understand the basic structure of the DNA double helix and the rules of base pairing.
Step 1: Understand the base pairing rule
In DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). This is known as complementary base pairing. Therefore, the number of adenine (A) bases will always equal the number of thymine (T) bases, and the number of guanine (G) bases will always equal the number of cytosine (C) bases.
Step 2: Determine the total number of nucleotide pairs
The DNA double helix in this problem is 100 nucleotide pairs long. Each nucleotide pair consists of two bases: one from the first strand and one from the second strand. Therefore, the total number of nucleotides is:
[
\text{Total nucleotides} = 100 \text{ nucleotide pairs} \times 2 \text{ nucleotides per pair} = 200 \text{ nucleotides}.
]
Step 3: Apply the given information
We are told that there are 25 adenine (A) bases. Since adenine pairs with thymine, the number of thymine (T) bases will also be 25. Therefore, the total number of adenine-thymine pairs is:
[
A = T = 25 \text{ pairs}.
]
Step 4: Determine the remaining bases
Since the total number of nucleotide pairs is 100, the remaining 100 nucleotide pairs must consist of guanine (G) and cytosine (C) bases. Since guanine pairs with cytosine, the number of guanine (G) bases must equal the number of cytosine (C) bases.
The total number of guanine-cytosine pairs is:
[
G = C = \frac{100 – 50}{2} = 25.
]
Thus, the number of guanine bases is 25.
Step 5: Final Answer
The DNA double helix contains 25 guanine bases.
Explanation:
DNA is made up of four nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C). These bases pair specifically, with adenine pairing with thymine, and guanine pairing with cytosine. The total number of pairs in a DNA molecule must reflect this complementary base pairing rule. By knowing the number of adenine bases, we can infer the number of guanine bases, which is equal to the number of cytosine bases. This ensures that the DNA molecule is balanced and follows the laws of base pairing.