Following are the criteria that a molecule must fulfil to act as a genetic material : $(i)$ It should be able to replicate itself. $(ii)$ It should be stable both chemically and structurally. $(iii)$ It should provide the scope for slow changes (mutation) which are required for evolution. $(iv)$ It should be able to express itself in the form of 'Mendelian Characters'.

According to these criteria, both $DNA$ and $RNA$ have the ability to direct their duplication (because of the rule of base pairing) and complementarity using specific processes whereas the other molecules in the living system fail to fulfil first criteria itself. eg. Protein.