A particular DNA double helix is 100 nucleotide pairs long and contains 25 adenine bases - VCE - SSCE Biology - Question 5 - 2012 - Paper 1

To determine the number of guanine bases in the DNA double helix, we need to apply the rules of base pairing in DNA. In DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).

1. Total nucleotide pairs: 100

2. Adenine bases (A): 25

3. Thymine bases (T): Since A pairs with T, there are also 25 thymine bases.

4. Remaining nucleotide pairs:

   Total nucleotide pairs = Number of A + Number of T + Number of G + Number of C

   Therefore:

   100 = 25 (A) + 25 (T) + Number of G + Number of C

   This simplifies to:

   100 = 50 + Number of G + Number of C

   Hence, Number of G + Number of C = 50.

5. Since G pairs with C: The quantity of G must equal the quantity of C. Thus, we can express:

   Number of G = Number of C = x

   Therefore:

   x + x = 50 ightarrow 2x = 50 ightarrow x = 25.

Thus, the number of guanine bases in this DNA double helix is 25. Therefore, the correct answer is:

A. 25.