The bond between thymine and adenine (T-A) and uracil and adenine (U-A) is a double bond, while the bond between cytosine and guanine (C-G) is a triple bond. This difference arises due to the specific hydrogen bonding patterns between these nucleotide bases.
In DNA, thymine (T) pairs with adenine (A) through two hydrogen bonds. Similarly, in RNA, uracil (U) replaces thymine and also pairs with adenine (A) through two hydrogen bonds. The structure of these bases allows for the formation of two hydrogen bonds: one between the nitrogen atom of thymine/uracil and the nitrogen atom of adenine, and another between the oxygen atom of thymine/uracil and the hydrogen atom of adenine.
On the other hand, cytosine (C) pairs with guanine (G) through three hydrogen bonds. The structure of cytosine and guanine allows for the formation of three hydrogen bonds: one between the nitrogen atom of cytosine and the oxygen atom of guanine, another between the oxygen atom of cytosine and the nitrogen atom of guanine, and a third between the nitrogen atom of cytosine and the hydrogen atom of guanine.
These specific hydrogen bonding patterns are crucial for the stability and specificity of DNA and RNA structures. The triple bond between cytosine and guanine provides additional stability compared to the double bond between thymine/uracil and adenine, which is why regions of DNA with higher C-G content are generally more stable and have higher melting temperatures.