They include cytosine , guanine , adenine , thymine in ( DNA ) , uracil in ( RNA ) and xanthine and hypoxanthine ( mutated forms of guanine and adenine ) . These are abbreviated as C , G , A , T , U , X and HX respectively . The primary nucleobases are cytosine , guanine , adenine ( DNA and RNA ) , thymine ( DNA ) and uracil ( RNA ) , abbreviated as C , G , A , T , and U , respectively .