If 35% of the bases in a region of the mouse genome are cytosine, what percentage in that region are adenine? 
 A. 15%

B. 20%

C. 30%

D. 35%

E. 40%

A. 15%

Explanation:

DNA is composed of four nitrogen bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). Chargaff's rule of base pairing states that A always pairs with T, while G always pairs with C. The total distributed percentage of the four 4 nitrogen bases must be equal to 100%. In this case, the C content is 35%, thereby G=35% (i.e., equal to C content), while the remaining 30% must be equally divided between T=15% and A=15%.