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 :
In a DNA double helix, the bases pair specifically: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). This is known as base pairing, and it follows the Chargaff’s rules. According to Chargaff’s rules, in a double-stranded DNA molecule:
- The amount of adenine (A) is equal to the amount of thymine (T).
- The amount of cytosine (C) is equal to the amount of guanine (G).
Problem Breakdown:
We are given:
- The total length of the DNA double helix = 100 nucleotide pairs.
- The number of adenine (A) bases = 25.
Since adenine pairs with thymine, the number of thymine (T) bases will also be 25.
Thus, the total number of A and T bases combined is:
[
\text{A} + \text{T} = 25 + 25 = 50 \text{ bases}.
]
The remaining 50 nucleotide pairs must consist of cytosine (C) and guanine (G), because the total number of nucleotide pairs is 100, and A/T account for 50 pairs.
According to Chargaff’s rule:
[
\text{C} = \text{G}.
]
Therefore, the 50 remaining bases must be split equally between cytosine and guanine. This gives:
[
\text{C} = \text{G} = \frac{50}{2} = 25.
]
So, the number of guanine (G) bases is 25.
Explanation:
The structure of DNA is such that the two strands are complementary. This means that for every adenine in one strand, there must be a thymine in the other strand, and for every cytosine, there must be a guanine. Chargaff’s rule ensures that the quantities of adenine and thymine, as well as cytosine and guanine, are equal in any double-stranded DNA molecule. Since we know that there are 25 adenine bases, it follows that there must be 25 thymine bases. The remaining bases must then be split equally between cytosine and guanine, giving us 25 guanine bases.
