Assume that a 100-base pair DNA double helix contains 45 cytosines. How many adenines are there?
The correct answer and explanation is:
In a DNA double helix, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). According to Chargaff’s rules, the amount of adenine equals the amount of thymine, and the amount of cytosine equals the amount of guanine in a given DNA molecule.
Given that the DNA molecule is 100 base pairs long and contains 45 cytosines, this means that there are also 45 guanines (since C pairs with G). The remaining base pairs must be adenine-thymine pairs. Since there are 100 base pairs in total, we subtract the 45 cytosines and 45 guanines from the total, which leaves us with 10 base pairs for adenine-thymine pairing.
Because the number of adenines is equal to the number of thymines, there must be 5 adenines and 5 thymines in the molecule.
Thus, there are 5 adenines in this DNA sequence.
Explanation:
- Total base pairs = 100
- Cytosine = 45
- Guanine = 45
- Adenine + Thymine = 100 – (45 + 45) = 10
- Adenine = Thymine = 10 / 2 = 5
This explanation follows from the structure of DNA and the way bases pair in complementary strands, ensuring the molecule is stable and functions properly in processes like replication and transcription.