Why is adenine paired with thymine

In this image you can see that the -NH and -OH groups of both Guanine and Cytosine are aligned and connect through hydrogen bridges.

This is a favourable situation for both molecules to be in, as both have 3 usable groups and no groups are in the way. Adenine and Thymine also have a favorable configuration for their bonds. When one pairs Adenine with Cytosine, the various groups are in each others way. For them to bond with each other would be chemically unfavorable. Why does adenine pair with thymine and not cytosine? Feb 11, Base stacking in this way creates a hydrophobic core on the DNA [6].

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Since they are not very strong bonds, they can be broken at elevated temperature. In DNA replication and transcription, the initiation of these reactions often starts at A-T rich sites because the breakage of two hydrogen bonds between A and T requires less energy than G-C rich sites which have three hydrogen bonds between G and C.

The two nitrogenous bases are held together by three hydrogen bonds. The first hydrogen bond is found between Oxygen atom of the keto group at C-2 of cytosine and one of the Hydrogen atom of the amino group at C-2 of guanine. The second hydrogen bond is formed between N-3 of cytosine and Hydrogen atom attached to N-1 of guanine.