A DNA double helix is 500 nucleotides long, so it contains a total of 1,000 nucleotides between the two strands of the double helix. If 300 of those nucleotides are adenine (A), how many would be guanine (G) g

200

Explanation:

According to Chargaff's rules, Adenine pairs with Thymine and Guanine pairs with Cytosine, so in a DNA double helix the number of A bases equals the number of T bases, and the number of G bases = C bases.

If 300 nucleotides are A, then 300 are also T.

A+T = 600 nucleotides out of 1000, so the remaining 400 must be G+C.

Because the number of G equals the number of C, there must be 200 Guanine nucleotides in that molecule.