A molecule that can act as genetic material must fulfill the following criteria:
i) It should chemically and structurally be stable.
ii) It should be able to generate its replica (replication).
iii) It should provide the scope for slow mutation that are required for evolution.
iv) It should be able to express itself in the form of Mendelian characters.
The genetic material should be stable enough not to change with different stages of life cycle, ago or with change in physiology of the organism. DNA being more stable is preferred as genetic material, as
a) Free 2'OH of RNA makes it more labile and easily degradable. Therefore DNA in comparison is more stable.
b) Presence of thymine (5 - Methyl uracil) at the place of uracil also confers additional stability to DNA.
c) RNA being unstable mutates at a faster rate.