mRNA is only short strand whereas DNA is two strands held together in a double helix, 3 dimensional shape. The strands are also much longer. mRNA is just contains an OH group which makes it unstable and more reactive and so more prone to mutations, because of its length its also able to travel through nuclear pores. Whereas the DNA cannot leave nucleus because it needs to be protected. Both of them contain phosphates, mRNA contain the sugar - ribose, whereas DNA contains the deroxybose sugar. Also in mRNA the base thymine doesn't exist, instead uracil is present. Whereas DNA has thymine, adenine, guanine and cytosine.
