AP BIOLOGY

Detailed explanation-1: -Adenine and guanine belong to the double-ringed class of molecules called purines. Cytosine, thymine, and uracil are all pyrimidines.

Detailed explanation-2: -Because of their structural similarity, we usually refer the nine-member double rings adenine and guanine as purines, and six-member single-ring thymine, uracil, and cytosine are pyrimidines.

Detailed explanation-3: -The two-carbon nitrogen ring bases (adenine and guanine) are purines, while the one-carbon nitrogen ring bases (thymine and cytosine) are pyrimidines.

Detailed explanation-4: -The two-carbon nitrogen ring bases adenine and guanine are purines, whereas the one-carbon nitrogen ring bases thymine and cytosine are pyrimidines. Hence statement 2 is false.

Detailed explanation-5: -Cytosine and thymine are the two major pyrimidine bases in DNA and base pair (see Watson–Crick Pairing) with guanine and adenine (see Purine Bases), respectively.