How does the function of mrna differ from dna?

DNA for one does not leave the nucleus. mRNA on the other hand is free in the cytoplasm.

DNA functions as the main code for the behaviour of the cell and for the production of all its . While it does not leave the nucleus, mRNA which is free can line up alongside DNA in the nucleus and recreate sections of the code to be replicated outside the nucleus. mRNA is then free to leave with this particular code and tRNA in the cytoplasm brings free amino acids to the mRNA code in order to construct new proteins on the surface of ribosomes.

DNA is the main rulebook, and messenger RNA relays this message into the cell where the rulings of the DNA can take place, via producing the proteins needed, etc.