Question

why RNA is not used in place of DNA to find criminal

Answer 1

Because DNA is DNA only..

Answer 2

DNA is commonly used because it is long lived and more stable than RNA. DNA is stable in alkaline conditions because deoxyribose sugar is less reactive due to C-H bond as compared to RNA which is not stable in alkaline conditions because ribose is more reactive due to C-OH bond. Moreover DNA has larger grooves which make it harder for enzymes to attack whereas RNA has smaller grooves which make it easy to be attacked by enzymes