Why do DNA bases pair

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Base pairing

The DNA can only serve as a store of information, as there is something called molecular recognition between the four nucleobases. Only the pairs match each other:

  • Adenine to thymine and
  • Guanine to cytosine.

In the DNA there is always a purine base opposite a pyrimidine base. How does this selectivity come about? In order to clarify this question, it is important to take a closer look at the spatial structure of the nucleic bases.

Tab. 1
3D representation and structural formula of some nucleic bases

This selectivity of the bases to one another, the so-called base pairing, is based on the hydrogen bridges that can form between the bases. The possible strength of these hydrogen bridges is decisively influenced by the spatial structure of the bases. The maximum possible simultaneous formation of several hydrogen bridges is only possible if the spatial arrangement is optimally matched. Even small deviations from the optimal geometry result in a very strong reduction in the bridge thickness. For example, the sum of the strength of the hydrogen bonds between thymine and guanine is much weaker than that between thymine and adenine. This strong difference in energy is due to the fact that hydrogen bridges are relatively weak. As a result, they are in a rapidly changing dynamic equilibrium and molecules therefore only associate with each other via several hydrogen bridges if the energies of this interaction exceed those of other arrangements in which only one bridge is formed. Several hydrogen bridges that are not ideally aligned can even be less stabilizing overall than a single bridge that is spatially optimally aligned.

The structure of the nucleic bases is such that only one base fits exactly to one other. Only with the pairs adenine to thymine and guanine to cytosine are particularly strong binding interactions via several hydrogen bridges optimally possible. There are two hydrogen bonds between adenine and thymine and even three with guanine and cytosine.

The matching base pairings are called complementary. In DNA, this selectivity of the bases to one another is the basis of their ability to store information. This selectivity of the bases makes it possible for the DNA to replicate and to read information from it.

Tab. 2
Selectable variants of the 3D representation

The sequence of the bases in the DNA encodes the blueprint for all proteins. The double strand can open and since the bases in the two strands are complementary to each other, one strand alone can act as a template. If the two DNA strands separate, each of the two single strands can be completed by the addition of the respective complementary bases to form an exact copy of the original DNA double helix. This process is known as replication.