6. A double helical DNA molecule is composed of 32 mol % of adenosine. The mol % of cytosine in this DNA molecule is ____.
Chargaff’s Rule: Calculating the Cytosine Percentage in Double-Stranded DNA
Introduction
The discovery of Chargaff’s rules was one of the most significant milestones in molecular biology because it provided the experimental evidence that ultimately led to the Watson and Crick model of DNA structure. Chargaff demonstrated that although the nucleotide composition varies among different organisms, certain relationships between the nitrogenous bases always remain constant in double-stranded DNA. Specifically, the amount of adenine always equals thymine, while the amount of guanine always equals cytosine. These relationships arise because adenine pairs exclusively with thymine through two hydrogen bonds, whereas guanine pairs exclusively with cytosine through three hydrogen bonds.
Correct Answer
Correct Answer: 18%
Detailed Explanation
Since the DNA molecule is double stranded, it follows Chargaff’s rule, which states that:
A = T
G = C
The total percentage of all four nitrogenous bases must equal 100%.
The question states that:
Adenine (A) = 32%
According to Chargaff’s rule:
Thymine (T) = 32%
Therefore,
A + T = 32 + 32 = 64%
The remaining percentage belongs to guanine and cytosine.
G + C = 100 − 64 = 36%
Since guanine and cytosine occur in equal amounts:
G = C = 36 ÷ 2 = 18%
Hence, the percentage of cytosine is 18%.
Step-by-Step Calculation
Step 1: Apply Chargaff’s Rule
A = T = 32%
Step 2: Calculate A + T
32 + 32 = 64%
Step 3: Calculate G + C
100 − 64 = 36%
Step 4: Divide Equally
C = G = 36 ÷ 2 = 18%
Final Calculation
| Base | Percentage |
|---|---|
| Adenine (A) | 32% |
| Thymine (T) | 32% |
| Guanine (G) | 18% |
| Cytosine (C) | 18% |
Chargaff’s Rules
| Rule | Explanation |
|---|---|
| A = T | Adenine always pairs with thymine. |
| G = C | Guanine always pairs with cytosine. |
| A + G = T + C | Purines equal pyrimidines. |
| Total Bases = 100% | The sum of all nucleotide percentages equals 100. |
Base Pairing in DNA
| Nitrogenous Base | Complementary Base | Hydrogen Bonds |
|---|---|---|
| Adenine (A) | Thymine (T) | 2 |
| Guanine (G) | Cytosine (C) | 3 |
Shortcut Formula for Similar Questions
Whenever the percentage of adenine or thymine is given:
C = G = (100 − 2A) ÷ 2
or
C = G = (100 − 2T) ÷ 2
Applying the formula:
C = (100 − 2 × 32) ÷ 2
= (100 − 64) ÷ 2
= 36 ÷ 2 = 18%
Biological Significance
Chargaff’s rules provided the experimental foundation for the Watson-Crick double helix model of DNA. Complementary base pairing ensures faithful DNA replication, accurate transmission of genetic information, efficient DNA repair, and proper gene expression. These principles remain fundamental to molecular biology, genomics, biotechnology, forensic science, DNA sequencing, and recombinant DNA technology.
Final Answer
A = 32%
T = 32%
G + C = 36%
C = 18%
Correct Answer: 18%
