mRNA aka messenger rna is what is created to "transcribe the dna". Since dna cannot leave the nucleus, it must somehow give its information a different way. What happens is that DNA opens and different nucleotides form together with the bases that are now open. This form is called mRNA. mRNA now takes this information, travels outside the nucleus to the ribisomes. Here the information is read and proteins are synthesized. In conclusion mRNA is needed to transfer the info out of the nucleus