AP BIOLOGY

Detailed explanation-1: -Guanine in the double helix pairs with cytosine, so you will see CG pairs; one on one strand and one on the other.

Detailed explanation-2: -The chemical structures of Thymine and Cytosine are smaller, while those of Adenine and Guanine are larger. Size and structure of the specific nucleotides cause Adenine and Thymine to always pair together while Cytosine and Guanine always pair together.

Detailed explanation-3: -In order to form this hydrogen bond, both bases need to twist in order to facilitate the hydrogen bond. This is an energetically expensive event, hence hydrogen bonds forming functional groups are not complementary in the case of adenine and cytosine. This is applicable to thymine and guanine too.

Detailed explanation-4: -The bases are the “letters” that spell out the genetic code. In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.