A DNA segment contains 100 Adenine and 100 cytosines, how many nucleotides are present in the segment?

The correct answer is 400.

To find the total number of nucleotides in the double-stranded DNA segment, we apply Chargaff's rules of complementary base pairing:
1. Adenine (A) always pairs with Thymine (T), meaning their quantities are equal:
$A=T$
2. Cytosine (C) always pairs with Guanine (G), meaning their quantities are equal:
$C=G$

From the given information:
- The number of Adenine (A) nucleotides = 100
- The number of Cytosine (C) nucleotides = 100

Using Chargaff's rules, we can determine the remaining bases:
- Since A = 100, the number of Thymine (T) nucleotides = 100.
- Since C = 100, the number of Guanine (G) nucleotides = 100.

The total number of nucleotides in the segment is the sum of all four bases:
$\text{Total Nucleotides}=A+T+G+C$

Substituting the values into the equation: