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.

Detailed explanation-2: -Within a double-stranded DNA molecule, adenine bases on one strand pair with thymine bases on the opposite strand.

Detailed explanation-3: -Reason: Adenine specifically forms hydrogen bonds with guanine whereas cytosine forms hydrogen bonds with thymine.

Detailed explanation-4: -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-5: -The answer has to do with hydrogen bonding that connects the bases and stabilizes the DNA molecule. The only pairs that can create hydrogen bonds in that space are adenine with thymine and cytosine with guanine. A and T form two hydrogen bonds while C and G form three.