Write a comparative note the properties of $DNA$ and $RNA.$
 

From Hershey - Chase experiment it become an established fact that it is $DNA$ that acts as genetic material.

However it subsequently become clear that in some viruses, $RNA$ is the genetic material. (for example, Tobacco Mosaic viruses, $QB$ bacteriophage etc.)

A molecule that can act as a genetic material must fulfill the following criteria: $(i)$ It should be able to generate its replica (Replication). $(ii)$ It should chemically and structurally be stable. $(iii)$ It should provide the scope for slow changes (mutation) that are required for evolution. $(iv)$ It should be able to express itself in the form of 'Mendelian Characters'.

If on examines each requirement one by one because of rule of base pairing and complementarity, both the nucleic acids ($DNA$ and $RNA$) have ability to direct their duplications.

The other molecules in the living system such as proteins fail to fulfill first criteria itself.

The genetic material should be stable enough not to change with different stages of 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 distroy 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, $2'$-$OH$ 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 nucleic acids the $DNA$ is a better genetic material.

- The presence of thymine at the place of uracil also confers additional stability to $DNA$.

Both $DNA$ and $RNA$ are able to mutate.

$RNA$ being unstable, mutate at a faster rate.

$-$ Viruses having $RNA$ genome and having shorter life span mutate and evolve faster.

- $RNA$ can directly code for the synthesis of proteins, hence can easily express the characters.

- $DNA$ is dependent on $RNA$ for synthesis of proteins.

The protein synthesising, machinery has evolved around $RNA$.

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.