During the course of evolution why $DNA$ was chosen over $RNA$ as genetic material ? Give reasons by first discussing the desired criteria in a molecule that can act as genetic material and in the light of biochemical differences between $DNA$ and $RNA$.

A molecule that can act as a genetic material must fulfill the following criteria:

$(i)$ It should be able to generate its replica (Replication): Because of rule of base pairing and complementarity, both the nucleic acids ($DNA$ and $RNA$) have the ability to direct their duplications. The other molecules in the living system, such as proteins fail to fulfill first criteria itself.

$(ii)$ It should chemically and structurally be stable.: The genetic material should be stable enough not to change with different stages of life cycle, age or with change in physiology of the organism. Stability as one of the properties of genetic material was very evident in Griffith's 'transforming principle' itself that heat, which killed the. bacteria,- at least did not destroy some of the properties of genetic material. This now can easily be explained in light of the $DNA$ that the two strands being complementary if separated by heating come together, when appropriate conditions are provided. Further, $2$ $'-OR$ group present at every nucleotide in $RNA$ is a reactive group and makes $RNA$ labile and easily degradable.$ RNA$ is also now known to be catalytic, hence reactive. Therefore, $DNA$chemically is less reactive and structurally more stable when compared to $RNA$. Therefore, among the two acids, the $DNA$ is a better genetic material. In fact, the presence of thymine at the place of un confers additional stability to $DNA.$

$(iii)$ It should provide the scope for slow changes (mutation) that are required for evolution: Both $DNA$ and $RNA$ are able to mutate. In fact, $RNA$ being unstable, mutate at a faster rate. Consequently, viruses having $RNA$ genome and having shorter life span mutate and evolve faster.

$(iv)$ It should be able to express itself in the form of 'Mendelian Characters' : $RNA$ can directly code for the synthesis of proteins, hence can easily express the characters. $DNA$, however, is dependent on $RNA$ for synthesis of proteins. The protein synthesising machinery has evolved around $RNA.$ The above discussion indicate that both $RNA$ and $DNA$ can function as genetic material, but $DNA$ being more stable is preferred for storage of genetic information. For the transmission of genetic information, $RNA$ is better. $RNA$ being a catalyst was reactive and hence unstable. Therefore, $DNA$ has evolved from $RNA$ with chemical modifications that make it more stable.