Question

Why do base pairs never self associate? A x A, U x U, C x C, G x G

Versus

A x T, G x C

Answer 1

it is necessary that smaller pyrimidines (T, C) base pair with larger purines (A, G) to maintain a diameter of 2nm across the double helix DNA. If two purines (A, G) were to base pair, the diameter would be larger than 2nm and if two pyrimidines(T,C) were base pair results in smaller diameter than 2nm. Hydrogen bonding is mainly responsible for holding the two DNA strands together. Adenine (A) is able to form two hydrogen bonds so it get associated with thymine(T), which also able to form two hydrogen bonds. Cytosine(C), which is similar to that of thymine(T), does not base-pair with adenine(A) because their hydrogen bonding does not line them correctly during association. Guanine(G) is able to form three hydrogen bonds so get line up correctly with cytosine(C), which also forms three hydrogen bonds during their association. Guanine(G) which is similar to that of adenine(A), does not base pair with thymine(T) because their hydrogen bonding would not line them correctly during association.

Therefore, base paring is possible only Iin case of A*T, G*C not in A*A, U*U. C*C, G*G.