Extra Insurance. Assume a 100 base pair DNA double helix contains 45 cytosine’s, how many adenines are there in this double helix? Explain how you got this number.
The correct answer and explanation is :
In a DNA double helix, base pairing follows Chargaff’s rules, which state that in a double-stranded DNA molecule, the number of adenine (A) residues is equal to the number of thymine (T) residues, and the number of cytosine (C) residues is equal to the number of guanine (G) residues. These pairings occur because of complementary base pairing: adenine always pairs with thymine, and cytosine always pairs with guanine.
Given that we have a 100 base pair (bp) DNA molecule and there are 45 cytosine (C) bases, let’s use this information to determine the number of adenines (A) in the molecule:
Step-by-Step Breakdown:
- Base Pairing Rules:
- Cytosine (C) pairs with guanine (G), so the number of cytosines (C) is equal to the number of guanines (G).
- Adenine (A) pairs with thymine (T), so the number of adenines (A) is equal to the number of thymines (T).
- Total Base Pairs:
- The total number of base pairs in the DNA molecule is 100.
- Cytosine and Guanine:
- There are 45 cytosines in the DNA sequence. Since cytosine pairs with guanine, there must also be 45 guanines.
- Remaining Base Pairs:
- The total number of base pairs is 100. So, the remaining base pairs must be adenine-thymine pairs.
- The sum of cytosine and guanine bases is 45 + 45 = 90 bases. This leaves 100 – 90 = 10 bases, which must be adenine-thymine pairs.
- Adenine and Thymine:
- Since adenine pairs with thymine, and the number of adenines is equal to the number of thymines, there must be 5 adenines and 5 thymines.
Conclusion:
Thus, there are 5 adenines in the given DNA double helix.
Explanation:
DNA is structured in a double helix, with two strands running antiparallel to each other. The strands are held together by hydrogen bonds between the nitrogenous bases: adenine pairs with thymine, and cytosine pairs with guanine. These pairings are crucial for the stability of the DNA molecule and its ability to replicate accurately. Chargaff’s rules help us predict the composition of the DNA based on the known pairings.
In this case, knowing the number of cytosines (45) allows us to infer that there are 45 guanines. With 100 total base pairs, subtracting the 90 bases that are cytosine-guanine pairs leaves us with 10 adenine-thymine pairs. Since the number of adenines must equal the number of thymines, the result is that there are 5 adenines and 5 thymines.