If a DNA double helix that is 100 base pairs in length has 32 adenines, how many cytosines, guanines, and thymines must it have?
The correct answer and explanation is :
To solve this question, we must understand base pairing rules in DNA and apply simple arithmetic.
🧬 Correct Answer:
- Thymine (T): 32
- Guanine (G): 36
- Cytosine (C): 36
🔍 Explanation (300+ words):
DNA (deoxyribonucleic acid) is a double-stranded molecule composed of two complementary strands that form a double helix. The building blocks of DNA are nucleotides, each containing a nitrogenous base. There are four types of bases in DNA:
- Adenine (A)
- Thymine (T)
- Guanine (G)
- Cytosine (C)
In DNA, the base pairing rules (known as Chargaff’s rules) state:
- A pairs with T (A = T)
- G pairs with C (G = C)
This means the amount of adenine will always be equal to thymine, and the amount of guanine will always be equal to cytosine in a double-stranded DNA molecule.
Now, let’s analyze the question:
- The DNA double helix has 100 base pairs, meaning there are 100 A-T pairs and G-C pairs total.
- Since each base pair includes two bases (one from each strand), the molecule contains 200 individual nucleotides.
- The number of base pairs = 100, so:
[
A-T \text{ pairs} + G-C \text{ pairs} = 100
]
We are told that the DNA has 32 adenines (A).
Since A pairs with T, there must also be 32 thymines (T).
So:
- A-T pairs = 32
- That accounts for 32 base pairs.
This means the remaining base pairs must be G-C pairs:
[
100 \text{ total base pairs} – 32 \text{ A-T pairs} = 68 \text{ G-C pairs}
]
Thus:
- Guanines (G) = 34
- Cytosines (C) = 34
Oops! Earlier, we said 36 of each — let’s fix that!
Correction:
- A = 32
- T = 32
- G = 34
- C = 34
✅ Final Answer:
- Thymine = 32
- Guanine = 34
- Cytosine = 34
The earlier values of 36 were a miscalculation — thanks for catching that!