A molecular biologist analyzes a DNA sample and finds that adenine comprises 28% of the nucleotides. In this double-stranded DNA sample, what percentage of nucleotides would you expect to be guanine, and what principle explains this relationship?
A. 22% guanine; purines equal pyrimidines
B. 28% guanine; each base pairs with itself in the double helix
C. 44% guanine; guanine must equal the sum of adenine and thymine
D. 72% guanine; guanine comprises all non-adenine nucleotides
Answer (1)
Adenine (A) is 28%, so thymine (T) is also 28%.
Total percentage of A and T: 28% + 28% = 56% .
Remaining percentage for guanine (G) and cytosine (C): 100% − 56% = 44% .
Guanine (G) percentage: 2 44% = 22% . The answer is 22% .
Explanation
Understanding the Problem We are given that adenine (A) makes up 28% of the nucleotides in a double-stranded DNA sample. We need to find the percentage of guanine (G) and the principle that explains this.
In double-stranded DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). This means the amount of A equals the amount of T, and the amount of G equals the amount of C.
Finding Thymine Percentage Since adenine (A) is 28%, thymine (T) is also 28%.
Calculating Total A+T Percentage The total percentage of adenine (A) and thymine (T) is: 28% + 28% = 56%
Calculating Total G+C Percentage The remaining percentage is for guanine (G) and cytosine (C): 100% − 56% = 44%
Finding Guanine Percentage and Stating the Principle Since guanine (G) equals cytosine (C), the percentage of guanine is: 2 44% = 22%
The principle that explains this relationship is that in double-stranded DNA, the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine (Chargaff's rules).
Examples
Understanding the base composition of DNA is crucial in various fields, such as forensic science and personalized medicine. For instance, in forensic science, DNA samples found at crime scenes are analyzed to identify individuals. Knowing the percentage of each nucleotide (A, T, G, C) can help confirm the identity or exclude suspects. In personalized medicine, understanding a patient's genetic makeup, including nucleotide percentages, can help tailor treatments to their specific needs, improving the effectiveness of therapies and minimizing side effects. This knowledge ensures accurate diagnoses and personalized treatment plans.
