DNA Base Pairing

Purines pair with pyrimidines due to steric constraints imposed by the DNA sugar-phosphate backbone. The available space for base pairs is 10.85 angstroms. Two purines together are too large to fit, while two pyrimidines create too large a gap for hydrogen bonds to form. A purine-pyrimidine pair fits perfectly and maintains the correct distance for hydrogen bond formation.

Adenine and thymine form two hydrogen bonds. Adenine has one hydrogen atom positioned close to an oxygen in thymine, and thymine has one hydrogen atom close to a nitrogen in adenine. These complementary arrangements allow the two hydrogen bonds to form at the optimal distance of approximately two angstroms.

Adenine-thymine pairs form two hydrogen bonds, while guanine-cytosine pairs form three hydrogen bonds. The additional hydrogen bonding in G-C pairs occurs because guanine has an oxygen and cytosine has both an oxygen and nitrogen positioned to form three hydrogen bonds. This makes G-C pairs more stable and requires more energy to separate during DNA replication.

Adenine cannot pair with cytosine because their hydrogen bonding sites are incompatible. Cytosine has a hydrogen atom where thymine has an oxygen, and cytosine lacks the nitrogen where thymine has a hydrogen. These misaligned positions prevent the formation of hydrogen bonds between adenine and cytosine.

Chargaff's rules state that in double-stranded DNA, adenine equals thymine, guanine equals cytosine, and the sum of purines equals the sum of pyrimidines. These rules reflect the complementary base pairing pattern where A pairs with T and G pairs with C. This equivalence maintains consistent DNA width and ensures faithful genetic information storage.

Base analogs such as acyclovir are molecules that replace standard DNA bases during replication. Acyclovir, a guanine analog, pairs with adenine normally but lacks a 3' end on its nucleotide. DNA polymerase cannot continue forming base pairs after incorporating acyclovir, terminating replication and preventing viral DNA synthesis in herpes infections.

Base pairing specificity results from the precise positioning of hydrogen bonding sites on each nitrogenous base and the steric constraints of the DNA helix. Only complementary bases have correctly aligned hydrogen atoms and electronegative atoms to form hydrogen bonds at the required two angstrom distance. DNA replication enzymes reinforce this specificity by ensuring only correct base pairs are incorporated.