Question

A single strand of DNA has an A+T/C+G ratio of 1.67. The ratio in the complementary strand would be

• A. 0.60
• B. 1.33
• C. 1.45
• D. 1.67

Hint:

While Chargaff's rules (\( A=T \) and \( G=C \)) apply to double-stranded DNA as a whole, they do not necessarily apply to a single strand of DNA.
However, the symmetric nature of base pairing guarantees that the ratio \( \frac{A+T}{C+G} \) is identical in both complementary strands.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Double-stranded DNA consists of two complementary, antiparallel polynucleotide chains.
According to Watson-Crick base-pairing rules, Adenine (A) always pairs with Thymine (T), and Cytosine (C) always pairs with Guanine (G).
Detailed Explanation:
Let us represent the bases of the first strand (Strand 1) as \( A_1, T_1, C_1, \text{ and } G_1 \).
The given ratio for Strand 1 is:
\[ \frac{A_1 + T_1}{C_1 + G_1} = 1.67 \] Let the bases on the complementary strand (Strand 2) be represented as \( A_2, T_2, C_2, \text{ and } G_2 \).
By base-pairing rules:
- \( A_2 = T_1 \)
- \( T_2 = A_1 \)
- \( C_2 = G_1 \)
- \( G_2 = C_1 \)
Now, let us write the ratio of interest for Strand 2:
\[ \text{Complementary Ratio} = \frac{A_2 + T_2}{C_2 + G_2} \] Substitute the equivalent base values from Strand 1 into this expression:
\[ \frac{A_2 + T_2}{C_2 + G_2} = \frac{T_1 + A_1}{G_1 + C_1} = \frac{A_1 + T_1}{C_1 + G_1} \] Since addition is commutative, the expression for the complementary strand is mathematically identical to the expression for the original strand.
Therefore:
\[ \text{Complementary Ratio} = 1.67 \] The ratio \( \frac{A+T}{C+G} \) remains identical in both complementary strands of a DNA molecule.

Step 2: Final Answer:
The ratio in the complementary strand is 1.67.