AP BIOLOGY

Detailed explanation-1: -Two purine nucleotides cannot pair together, same with the pyrimidines.

Detailed explanation-2: -Introduction. Due to the presence of abundant hydrogen-bonding donors and acceptors, purine bases, especially guanine, can sometimes pair with another purine base to form purine·purine “mispair”.

Detailed explanation-3: -In naked unmodified nucleic acid duplexes, purines (guanine and adenine) pair up with pyrimidines (cytosine, thymine, and uracil) through complementary hydrogen bonds to form canonical G•C, A•T, and A•U Watson-Crick (WC) base-pairs (Figure 1A).

Detailed explanation-4: -Purines can’t combine with other purines due to double-ring structures. The higher the bond specificity higher the stability and the bond will be stronger. DNA hydrogen bond structures that cause only adenine to pair with thymine.

Detailed explanation-5: -For the two strands of the double helix to fit neatly, a pyrimidine must always be paired with a purine. The second thing you should notice in Figure 15.2. 3 is that the correct pairing enables formation of three instances of hydrogen bonding between guanine and cytosine and two between adenine and thymine.