Enzymes physically bring together molecules to help them bond more easily. This lowers the of the reaction, which is the amount of energy needed for the reaction to take place.

before it leaves the nucleus of a eukaryotic cell, mrna is altered in the following ways: - a cap is added to the 5' end of the mrna molecule; - a poly a tail is added to the 3' end; - introns are removed and exons are spliced together.