Describe in short about $RNA.$

 $RNA$ : A nucleotide has three components -a nitrogenous base, a pentose sugar (ribose in case of $RNA$ , and deoxyribose for $DNA$ ), and a phosphate group. 

$2'-OH$ 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.

$RNA$ was the first genetic material. There is now enough evidence to suggest that essential life processes (such as metabolism, translation, splicing, etc.), evolved around $RNA$.

$RNA$ used to act as a genetic material as well as a catalyst (there are some important biochemical reactions in living systems that are catalysed by $RNA$ catalysts and not by protein enzymes). But, $RNA$ being a catalyst was reactive and hence unstable.

Therefore, $DNA$ has evolved from $RNA$ with chemical modifications that make it more stable. $DNA$ being double stranded and having complementary strand further resists changes by evolving a process of repair.