AP BIOLOGY

Detailed explanation-1: -Under normal circumstances, the nitrogen-containing bases adenine (A) and thymine (T) pair together, and cytosine (C) and guanine (G) pair together. The binding of these base pairs forms the structure of DNA.

Detailed explanation-2: -The four bases that make up this code are adenine (A), thymine (T), guanine (G) and cytosine (C). Bases pair off together in a double helix structure, these pairs being A and T, and C and G. RNA doesn’t contain thymine bases, replacing them with uracil bases (U), which pair to adenine1.

Detailed explanation-3: -In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine. Source: definition from the National Human Genome Research Institute (NHGRI) Glossary of Genetic Terms. In addition to the letters A, C, T, and G, a number of other abbreviations can be used to represent the nucleotide bases.

Detailed explanation-4: -A always pairs with T and G always pairs with C because these are the only combinations that allow for hydrogen bonding to occur, given the spatial constraints of the double helix, which requires there to be one purine and one pyrimidine in each base pair.

Detailed explanation-5: -Hence, guanine pairs with cytosine by formation of three hydrogen bonds.